Weird move to do this now. Although maybe Microsoft did try or have been planning for this many years ago by now.
Because back in 2016 to 2017 would have made more sense to me due to the extreme popularity of Overwatch (especially the first year, it was a honeymoon story with the public). However, by now, World of WarCraft keeps losing players (and its expansions aren't good at all from what I hear, plus have a bunch of 'pay to win' issues in them where you can buy services and shortcut your way through content or something along the lines), Overwatch 1 in essentially is survival mode since 2 years (barely any new significant content, only surviving from Seasonal events skins and emotes and minor updates), StarCraft as a franchise at this point is all bug forgotten (maybe except in South Korea, even then maybe there too it's been slowing down a lot for a few years), Diablo II Resurrected did well but as of now the dust fell and the initial rush of players to check it out is over. Then we have Diablo 3 which never really got any significant new content either, only relying on every 3 to 4 months with Seasons that are within the 25+ in number of occurrences as of this typing. And I suppose I could add Heroes of the Storm in there just for the laughs (a game that's been literally abandoned, is only updated once every 3 to 4 months, and is supported only by a skeleton crew that takes care of the 'Legacy' games such as the original D2 and StarCraft 1).
I mean... yeah, I just find the timing weird. It just so happens that their 'interest' (Microsoft's) in buying them coincidences with basically the Titanic sinking process, rather than when it left the port.
If anything, Microsoft's priority will be to rejuvenate (and in some cases pretty much revive) those games before doing anything with them.