I wouldnt say its babys first arpg, I feel thats just belittling it for belittling sake, I do however agree with you that its targetting a casual arpg audience. Its certainly not as "spreadsheety" as PoE for example. In MMO terms, if Diablo is World of Warcraft, PoE is Eve Online, which is fine, some people prefer WoW, some people prefer Eve Online. I played Diablo 3 for years and that was more shallow than D4 seems to be so I'm sure I will get years out of D4, however I am that casual arpg player target that you mention. I've got far too many titles to play now and in the future to be dedicating 13,000 hours to 1 title.
Thats why I say that making it a PoE vs D4 vs LE vs Grim Dawn argument is fundamentally flawed because they play differently and are aimed at different audiences.