They can stick a 2 on it if they want, but the ingame play timer never reset, going from 6v6 to 5v5 isn’t such a big change to say its a sequel imo. Everything that was going to make 2 “2” was canceled. The large majority of characters, levels, cosmetics, modes, etc etc is the exact same. They just needed an excuse to start charging for skins. Blizzard sucks but its what the people I talk to play so its what I play.
Also not sure why it isnt in your library, but part of the “2” change was going free to play, so you can always redownload it. Unless you are saying OG Overwatch is gone and OW2 is there now, in which case yes they just updated the first game, there is no new game.