Actually this isn't an effect of AOE4 at all.Mbl's recent ladder opponents (last 25 games or so):
Dragonstar (x2), Kingstone, Overtaken, Miguel, FreakinAndy, DonCabron, Mr. Greed, Kongen 42 (x3), m0re, Unknown (Sebastian?) (x6), SongSong, TheBloodless, BamBam, Margougou (x3), Eren, Dogao
The ladder has never recovered from aoe4 release and it's pretty sad actually.
Pretty much all the top 15-20 except Hera are still mostly (if not exclusively) playing AOE2 .
It's just that most pros are now playing black forest, nomad or ranked team games. Rather than 1v1
Rage Forest especially is very popular right now.
I think this is a great thing. Team games are more enjoyable/entertaining IMO. Especially the 4v4 rage forest, which has everything from clown dark age to large scale post imp battles.
They're gonna bring more players and viewers into AOE