1) the release of the HD and Definitive Editions of the game, with graphics and gameplay that still hold up
2) availability on popular game stores like Steam (though it's a Microsoft game)
3) continuous balance patches, content updates, and occasional DLC
4) talented Twitch streamers and YouTubers casting games, commentary, and analysis (See T90Official and Spirit of the Law)
5) an active, mature, and welcoming player base on Reddit, Twitch, Discord, etc
6) frequent and varied tournaments with increasing prize pools (the recent Hidden Cup had the highest prize pool for an AoE2 tournament in 20 years)
Certainly in the context of talking about a publisher like Blizzard nothing but raw profit is of any concern. Arty ambitions is the something for the game studios that appear and die off, not for the publisher that seeks to endure.
SC2 did something very smart with its last expansion with the coop mode, and it became very popular despite being only sort of fleshed out. I could easily see that kind of game mode making for a hit RTS if it was the central focus, and had the standard MMO-lite stuff like loot and missions and raids and story and customizable units/armies and support for other group sizes.
They are focusing on games that can monetize user-base actively via micro-transactions and subscriptions RTS games are not really capable of this.
Last example i can think of is CoH 2, which was disappointing and the 'pay to win' commanders drove me away from it quickly. Plus i think original CoH is much superior game in terms of balance and fun.