They would and every man jack of us and woman jill of us knows it.
The anime has explicitly stated that Pikachu is unable to Mega Evolve since it is not a fully evolved Pokemon.
Pikachu Mega Evolve!Dedenne Primordial Regression!Uwaaaah!Primordial DedenneUwaaaah!Mega PikachuDedenne Mega Evolve!Are you having a nightmare?Raichu Ω αUwaaaaah!Mega Dedenne...Okie dokie, I'll accompany you so you can sleep peacefully.