That is a hard one. With niche RTS games, splitting the playerbase is absolutely a dangerous move in that sense. If anything, it is a rarity that players manage to maintain a community over older games, but it isn’t random either. It usually comes down to leaps of design.
For instance, AoE3 is wildly different to AoE2, so it is understandable that someone who can’t get into all the new AoE3 mechanics would just stick with AoE2 instead. But, what if they hadn’t released AoE3, but instead AoE2.1? AoE2.2? AoE2.3? At what point will they want to seek a reversal of all the problems that was introduced (as well as all the improvements)?
Personally I still hold the opinion that there were some better things about the original AoE2 over the Definitive Edition, despite how successful it was. I dislike automatic seeding, and think that DE has actually regressed in some ways like Pathfinding. I also find the live constant shift in balancing less interesting, as I played AoE2 for the sake of improving mechanics and strategies rather than the metaslaving of constantly only playing new, stronger “balanced” mechanics.
It is in that way that games like SC1BW are still charming, as minimal changes means players can keep iterating strategies in an effort to overcome weaknesses of their faction and the strength of others. And while this may not sound as interesting in of itself, there is another perk; that no one is live tweaking balance at any and every outcry of players online who don’t like the game to begin with, or who want to change it to something entirely differently.
Because that is the issue ultimately. Players aren’t developers. They WILL vote against their own interests again and again, because they don’t understand nor care for the underlying design of it all. They will ask for more, and more, and more, until it all gets boring and meaningless, at which point they turn around and call the game boring.
Anyway. Will a player leave because they changed the stats of 1 unit? Because they changed the pathfinding? Because of autoseeding? Because they added autoscouting? Autoqueue? Eventually the salimi slicing will reveal its damage, and eventually the original game will seem very different to the live one.
As for the question of why players don’t simply go back to the original? Well, maybe they will, but the realistic answer is that the biggest portion of playerbases are never hardcore fans or supporters. They will stick with whatever is the latest slop, until they themselves get bored. When you migrate that population from AoM to AoMDE then make AoMDE dogwater, you’ve killed AoM whether you intended to or not.