Microsoft has a lot of smart people working for it. Those people would not have agreed to buy ATVI at a 47% premium solely to make slight tweaks to ATVI's current library of game titles. For the acquisition to make sense at this price point, there must be something else that ATVI offers MS that MS doesn't already have (or is struggling to develop). What is that? It's probably MS betting on the
"metaverse" .
What is the most "metaverse" item in ATVI's library? Probably WoW with its "persistent virtual worlds that continue to exist even when you're not playing". Despite all the troubles with WoW, it still remains in a relatively unique place. It has managed to maintain millions of subscribers despite being 16+ years old (granted it has lost millions too). WoW is really unrivaled in this level of staying power.
When I don't log into COD for a month, it doesn't really matter. I can jump right back into the game and I haven't missed a thing. With WoW (particularly freshly after an expansion), when you can't log in for a month, you are missing out on progression and events and your friends and guildees' activities and progress.
So imagine taking that a step further. Having the worlds of flight simulator, Azeroth, Call of Duty, halo, diablo, etc, being persistent and ongoing - perhaps even interlinked to some extent. The worlds of those games keep evolving and changing even when you are not logged in. That would drive an entirely new kind of "desire" for game-players to engage with those titles (and perhaps the combined interlinked worlds). People fundamentally don't want to miss-out, so they will log in more.
Its that sort of experience from ATVI that makes them an attractive acquisition for MS. The value of the current games library is just the cherry on top. The real meal for MS is buying the talent that can make MS' vision of the metaverse a reality.