Okay, allow me to explain. Yes, that's something Microsoft is likely to do if they purchase it. They'll bake it into the core Windows Operating System by default. What is lost by that? Windows gamers already use it. They want it. They have it installed. Microsoft is doing nothing but saving you a chore.
The largest group who stand to be angry about this are NOT gamers, but people who would perceive Steam itself as "bloat", and don't want it included by default on their pristine machine out-of-the-box. Furthermore, unlike all the crap bloatware from manufacturers like Lenovo, Asus, etc., there's nothing about Steam requiring that it use system resources as part of the boot process, and regardless, it's easy to turn this off.
Since storage requirements are NOT a major concern on desktops, unlike tablets or phones, there's really nothing of consequence impacting users as a matter of "bloat". It will simply supplant the crappy little "MS Games" branch of the Microsoft Store as their default games software depository.
The only other group-- the principal group-- who stand to be upset are Linux gamers because if Microsoft buys Steam, they likely won't be keen to maintain & update its Linux compatibility, so gamers who avoided buying Windows, or prefer to use Linux for whatever reason, will suddenly be left in the lurch. But really, who gives a fuck about Linux gaming? As I just pointed out, the Linux/SteamOS Steam library is only 6.4% the size of the Windows Steam library.
Furthermore, virtually every single one of those games performs better on Windows-- in terms of framerate or stability-- than on Linux (because every game developer in existence is devoting the lion's share of their time to optimizing for Windows: their actual market).