Acti-Blizz already decided against doing another MMO, years ago. The reason was that their read of the online gaming market didn’t support making another MMO. The growth in online gaming was moving away from MMOs and towards other genres, like royales and MOBAs and team shooters and so on.
MMOs cost an epic amount of time and money to create. That is even more the case for a company like Blizzard, who would release any game with very high production values and a minimum of bugs (regardless of whether the design is good or not). And Acti-Blizz aren’t interested in that effort unless there’s a large payoff at the end – they only do blockbuster type games, not games that are merely “successful”. All of that ads up to deciding not to make another MMO, and it seems extremely doubtful that Blizzard will revisit that decision.
It makes far more sense, from a business/product perspective, to keep on producing iterations of WoW as long as there are enough subscribers to achieve the profitability level Acti-Blizz is looking for to justify continuing with the game.
As for the passion … Acti-Blizz is a public company. It’s about product, revenues and profits … first, last, and in the middle.