Actual Unpopular Opinon: No, both games cannot co-exist

Don't be deluded, without developer and sponsors' support the game will quickly fade away to a state where there is only one active twitch channel - maintained by a romanian grandpa and a spanish billager. And 100 people watching. We had been there already, even if you don't remember this

No, the new game will not attract new audience since you know... it is a mediocre game with crappy visuals in a niche genre. They obviously don't aim to attract new people, they aim to attract you. And since this is not gonna work by fair competition, they will push you to switch to aoe4 simply by suffocating aoe2 in every possible way. In the very best case scenario it is gonna be the same as civ5/civ6 story, in the worst - one game will die. Only one because another gonna be a stillborn