Valve isn't publicly traded, and while that doesn't make them 100 % trustworthy, I'd certainly give them the benefit of the doubt over Microsoft. They make a shit ton of money, but they aren't obligated to squeeze every last penny out of their customers.

Sure, it would be nice to have 1-click installers for other launchers within Steam and then automatically list games of other launchers in the Steam library, but I don't even think Valve is the primary blocker here.

Steam had the ability to add external shortcuts as long as I can remember, tools like Steam ROM Manager make use of that already.

Tim Sweeney for example always likes to cry to the press how bad monopolies are with Steam and their 30 % cut and whatnot, he doesn't seem to grasp that part of the reason almost nobody uses Epic Games Launcher is that it quite frankly sucks. Where is your multi-platform launcher? Ah, need Heroic to do the heavy lifting for you. They could offer an official Flatpak on the Steam Deck, easily installed from the Discover store, that uses Proton for games compatibility, is usable with a controller and adds itself to Steam (and when installing games, adds them to Steam as well).

I'm not that bothered by Valve having that kind of "grip on the market" because in my opinion they are also by far the best so I'd rather they own the biggest piece of the cake than anyone else.