That is literally what they’ve done. The default is CS2, but you can select a beta version in steam which enables CS:GO again. Matchmaking servers are all migrated to CS2, ofcourse, but community servers still work.

The reason they replaced CS:GO with CS2 instead of creating a seperate game is to not split the playerbase. Back when CS:Source released, the playerbase was essentially split in half, with many choosing to remain on CS 1.6, and it took a lot of effort to make CS:GO the standard.