Btw, I think that AoE 4 won’t ever replace AoE 2 (or 1, 3 and Myth).
The simple reason for that is that all Age games have their audience, with some overlap in playership. In my opinion, the “rivalry” between the games only exists in the minds of players who only play one game of the series extensively thinking that “their” game is the best and the others are bad.
I also don’t think AoE 2’s ongoing success is soely/almost exclusively based on nostalgia. That may be true for the CD version and HD Edition, which still have their playership as they don’t like the changes DE brought.
Definitive actually brought new players into the game, so nostalgia plays a rather negligible role. 2’s playership is also far away from retirement age as most players are in their mid 20s to 30s, so very much like the other Age games.
Nostalgia, however, is something that swings with every Age title - with AoE 4 for example, the marketing was built on nostalgia, as with Joan of Arc and Genghis Khan, two of the most popular characters from AoE 2 were placed on the cover of AoE 4.