To keep thousands of now irrelevant reviews, allow themselves to remonetize content and jumpstart a player base that now has no other choice but to switch.

CSGO is still in CS2's beta channel on Steam as csgo_demo_viewer or something like that, but obviously limited to community servers/offline play, no official dedicated servers.

to prevent fragmenting the player base

May have something to do with keeping the appid the same. My guess is this was done because of how skins work on the backend. I don't know for sure though. They should have done more testing on Linux however. It will be interesting to see what they do going forward.

It IS playable on Linux. It ISN'T playable on computers over ~10 years old.