MS only sponsored aoe2 for about 325k in 2021 if i looked right ,only S-tier checked .Half money was given to RBWs and it is possible MS only gave money because RB came to scene(meaning this money is not necessary expected unless other big company comes to host tournaments). I estimated 50k for KOTD4 as prinztech and MsHuggah sponsored it too. At this point of aoe4 timeline Aoe2 has got more money from MS than aoe4 (N4C is in march so not counted)That is the very crux of the matter. Aoe2 had over $600k in prize money in 2021...will that grow this year or has the pie stayed the same size and now aoe4 gets a sizable chunk of that? We can only wait and see since we have no tournament calendar and almost zero communication regarding the year ahead.
Let me turn this question around on you a little bit: if these RTS "legends" (as so many are quick to describe them as) weren't earning any money from the games they played (and with the exception of marinelord they were not), why didn't they dip their toes into aoe2 and try to compete there? Probably because it would take 15+ months of dedication to even sniff the top 16 whereas with aoe4 they can jump in and compete right away because it's a brand new game.
another answer: We are talking about growth not being as big because some money is going to aoe4. they never considered going pro or full time stream it before aoe4 release, they only came after aoe4 was launched meaning aoe2 wasn't relevant enough for them. Instead Aoe4 got them to here and to stay playing AoE bringing new audience with then which is only net positive as they would most likely play other games without aoe4. Aoe2 hasn't really got new top caster or pro in years(no growth on top).