Cross platform multiplayer isn’t held back by Steamworks. It’s held back, mostly, by Sony and Nintendo. You notice that PlayStation and Nintendo are the only platforms that regularly do not allow cross platform play. Even in games that do have cross platform play, Playstation usually only allows it with other consoles and not PC. Because Sony is fucking stupid.
I don’t even know wtf you mean by “cross launcher.” Every source of the game on a PC has always been able to connect with other players who got the game on different stores. IE Steam players can play with EGS players and Origin players just the same. If you just mean launching the game from one launcher rather than another… Steam lets you do this! You can add literally any fucking program to Steam and have the benefit of the overlay and even controller support when the game doesn’t have it native. Can’t say that about any of the rest of them.