Their whole idea with providing us with DE version of each game up until the release of AoE4 is to hype us for that game. If AoE4 was never made, DE versions most likely wouldn’t have come in the first place.
Therefor if AoE4 fails majorly then it’s common sense for them to stop caring for the other older titles as Microsoft will pull it off from the table and focus the resources elsewhere. And if this happens other companies that might have wanted to try and resurrect this genre won’t do it as they will see that as not worth the trouble.