Repeat universe until death, which will never come, so just keep remaking the universe and have fun. The Immortal plan to keep themselves entertained.
That makes little sense, however. Human history, as Eirin herself explained, divided by her history may as well be zero. These characters have many millions or even billions of years of history (if we don't count time they didn't actively experience due to relativistic speeds), and they're spending it all to try to recapture the couple decades they can co-occupy with these unaltered humans. Even if we count it as "they want to spend their lives with humans", then presuming a humanity that went extinct long before the sun exploded, they are still talking about a mere 10^4 to 10^6 years of humanity to spend time with between 10^10 to 10^14 year spans. (Or in other words, they spend 99.9999% of their lives just trying to find each other and re-enact a cycle that they can only enjoy with 0.0001% of their lives... each one of which would likely be more meaningless and boring than the last.)
That also ignores the whole problem of, you know, there now being another duplicate of each Hourai elixir character per cycle... and if we're talking about a sequence of thinking about what happens when you say "forever" and really mean forever, then that means the universe will be made of a black hole made of Kaguya-Moukou-Eirin paste.
That, however, also presumes the absolutely ludicrous premise that this is a perfect copy of the previous universe... Remember, they're creating this universe by just blindly smashing an impossibly fast force into a singularity containing all the matter in the universe. There's about ZERO chance they have any control over the results of what they're doing, and that's not even starting on questions of whether the same universal constants would even apply in the next universe, or, for that matter, that they'd have any chance of finding that next Earth after smashing their spaceship into the Big Bang and being rendered into subatomic particles literally spread the entire width of the whole new universe. (You thought finding each other after the Earth was smashed by a meteor would take a long time? Try the whole universe.) I can kind of put this one on hold just by accepting the handwave bullshit that Remi somehow fated THIS, too, and write that one off as just being magic, though. The rest of the plot flaws don't have magic as a defense.
Shouldn't they, you know, have higher goals than this?
Yeah, sure, they theoretically bought themselves time with this, but they've literally had billions of years to think of a plan, and hitting the reset button was all they came up with. (Although I blame that on timeskips where we are expected to believe characters did absolutely nothing but walk around a molten planet surface for a billion years.)
Gensoukyou is also a world where you can physically walk to the lands of the dead, meet the judge of the dead, have a conversation, and have the similarly eternal cycle of death and rebirth explained to you in detail by the psychopomps who work the system. Plus, you can have a tour of Heaven or all the Hells, and even pick up souvenirs at the gift shop. Either these characters should accept the cycle of life and death as it is, and develop a way to return to it, (I.E. counteract the Hourai Elixir,) or they should be working on devising a cycle of life and death and/or plane of existence they prefer more. (Ask Vivec for more details.) They're clearly at a level where they can bend reality to their whims, and any power they might need but don't yet have can be found through bending reality just a little more to have a new fate/universe where the random bullshit superpower they need does exist in Gensoukyou.
At this point, they're just stopping 10 turtles down; that's hardly enough to go all the way to infinity.
That makes little sense, however. Human history, as Eirin herself explained, divided by her history may as well be zero. These characters have many millions or even billions of years of history (if we don't count time they didn't actively experience due to relativistic speeds), and they're spending it all to try to recapture the couple decades they can co-occupy with these unaltered humans. Even if we count it as "they want to spend their lives with humans", then presuming a humanity that went extinct long before the sun exploded, they are still talking about a mere 10^4 to 10^6 years of humanity to spend time with between 10^10 to 10^14 year spans. (Or in other words, they spend 99.9999% of their lives just trying to find each other and re-enact a cycle that they can only enjoy with 0.0001% of their lives... each one of which would likely be more meaningless and boring than the last.)
Is it really that farfetched to believe that a couple of decades living with this bunch of non-Hourai humans is more positively exciting for the Hourai trio than either the many centuries/millennia they experienced before they ended up in Gensokyo, or the aeons said trio were forced to spend with nobody else other than themselves alive?
That also ignores the whole problem of, you know, there now being another duplicate of each Hourai elixir character per cycle... and if we're talking about a sequence of thinking about what happens when you say "forever" and really mean forever, then that means the universe will be made of a black hole made of Kaguya-Moukou-Eirin paste.
That, however, also presumes the absolutely ludicrous premise that this is a perfect copy of the previous universe... Remember, they're creating this universe by just blindly smashing an impossibly fast force into a singularity containing all the matter in the universe. There's about ZERO chance they have any control over the results of what they're doing, and that's not even starting on questions of whether the same universal constants would even apply in the next universe, or, for that matter, that they'd have any chance of finding that next Earth after smashing their spaceship into the Big Bang and being rendered into subatomic particles literally spread the entire width of the whole new universe. (You thought finding each other after the Earth was smashed by a meteor would take a long time? Try the whole universe.) I can kind of put this one on hold just by accepting the handwave bullshit that Remi somehow fated THIS, too, and write that one off as just being magic, though. The rest of the plot flaws don't have magic as a defense.
They didn't literally create a new universe while leaving the old one on the wayside. They restarted the same universe that they had existed in as it reverted to the pre-Big Bang singularity state -- with Eirin explicitly managing the whole process (as it was stated several times it the story), rather than leaving it up to chance; Eirin effectively played the role of God in this event.
And in the following pages, it's shown that the Hourai trio reincarnated into this new universe with no evidence of there ever having been any duplicates of them walking around, thus implying that all of the non-Hourai humans' souls were similarly reincarnated but without any memories whatsoever (barring a certain someone).
Shouldn't they, you know, have higher goals than this?
Yeah, sure, they theoretically bought themselves time with this, but they've literally had billions of years to think of a plan, and hitting the reset button was all they came up with. (Although I blame that on timeskips where we are expected to believe characters did absolutely nothing but walk around a molten planet surface for a billion years.)
Gensoukyou is also a world where you can physically walk to the lands of the dead, meet the judge of the dead, have a conversation, and have the similarly eternal cycle of death and rebirth explained to you in detail by the psychopomps who work the system. Plus, you can have a tour of Heaven or all the Hells, and even pick up souvenirs at the gift shop. Either these characters should accept the cycle of life and death as it is, and develop a way to return to it, (I.E. counteract the Hourai Elixir,) or they should be working on devising a cycle of life and death and/or plane of existence they prefer more. (Ask Vivec for more details.) They're clearly at a level where they can bend reality to their whims, and any power they might need but don't yet have can be found through bending reality just a little more to have a new fate/universe where the random bullshit superpower they need does exist in Gensoukyou.
At this point, they're just stopping 10 turtles down; that's hardly enough to go all the way to infinity.
You have to remember that a lot if not all of the supernatural stuff in both Gensokyo and related realms is dependent on human belief to exist; hell, Gensokyo was created in the first place to serve as a haven for youkai and other supernatural creatures once it became clear that human belief in their existence was on a terminal decline that will inevitably end with said creatures vanishing into nothingness one after another.
Now consider the fact that humanity had apparently either gone extinct or had completely abandoned/relocated from certain areas of the planet at around 600,000,000 million AD (the first flashback scenes), if the desert-like nature of what is obviously Honshu (that's where Gensokyo is physically located, after all) is any indication.