I put it back to "Please do not worry about me". Part of Koizumi's character is that he keeps a respectful distance between himself and the rest of the SOS Brigade, and his language is unusually polite, especially for a high school student. (Except for the fact that he calls Kyon "anata" all of the time.)
It's true that that particular sentence is formal but not unusually formal. But, the tone in general should be conveyed somehow.
As such, I am only able to provide a temporary fix to the problem.Please do not worry about me.I'll do it.Between several hundred and ten thousand years.You... Can that even be called aftereffects? Really...Wouldn't that be the future arc already!!...I can trust you, right?Nagato.This feeling of yours arose because Suzumiya Haruhi wished for it.For an hour, the feeling will subside....I knew it, somebody was being made into a scapegoat, and that was me.Of course.What about you, Koizumi? Since this is the case, wouldn't you want to join...That is true, but because Mori-san is waiting for my return today, I'm afraid that I will have to decline...Ahhh dammit.If you could do such a thing, I sure wish you had said it earlier.To resist this will result in problems occurring in Suzumiya Haruhi's mental state.Can't I just get a normal cleaning after all?There will be no problems.I see... Well, that's too bad.