I agree that the epic launcher sucks, but Steamworks has also refused crossplay forever (both cross platform between PC and consoles, and cross launcher on PC, which is why a lot of the not ancient games on gog didn't have multiplayer), meanwhile EOS gets you cross platform and cross launcher crossplay support. Pretty much anyone who wasn't a huge AAA dev used steamworks for multiplayer until epic launched eos.
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.
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.
Incorrect. Games that use steamworks for multiplayer can only play with other steam users. This has been an issue for people buying games on gog for over a decade. Big AAA devs use their own multiplayer backends, but most AA and indie use steamworks (or eos these days).
Steamworks doesn't support crossplatform or crosslauncher multiplayer, period. Every game that has cross-launcher play between steam and eg. gamepass, gog, epic, etc. has it because they either didn't use steamworks for multiplayer, or they replaced steamworks multiplayer for either EOS, GoG Galaxy, or their own custom multiplayer backend. This is not up for debate, this is something anyone even somewhat familiar with steamworks or gog would already know.
Homeworld Remastered only recently got cross-launcher multiplayer when they replaced steamworks with their shift backend. Same for Homeworld Deserts of Kharak.
Deep Rock Galactic doesn't support cross-launcher play or crossplay between the gamepass (xbox or pc) and steam versions because the steam version uses steamworks multiplayer. DRG support crossplay between the MS store and xbox console versions.
Brutal Legend straight up doesn't have multiplayer on GoG because it relied on steamworks and they didn't bother implementing anything for gog, so they just removed it.
Off the top of my head, No Man's Sky, Stellaris, Grim Dawn, and Darktide all had to write their own multiplayer backends to replace steamworks to get cross launcher play.
Literally every multiplayer game that was steam exclusive and then releases on epic removes steamworks and replaces it with EOS because steamworks doesn't support cross-launcher play (eg. Rising Storm 2, Mordhau)
edit: GOG docs that specifically point out that steamworks doesn't support crossplay.
In the future you should do literally any research to avoid making an ass of yourself.