Just for people to acknowledge that Steam is unethical and that we can do better. That's it.
If Valve suddenly decided tomorrow to release all of their source code on Github, all you’d get is a big blob of source code that is purpose built for Valve themselves and not really modular. They’d have so much technical debt and auditing requirements that it’d probably be easier to start from scratch, which I don’t think it’s reasonable to expect them to do.
You could make the same excuse for any company. Releasing the code under a Free Software license is all that's needed. Even if it's hard to compile (but it has to be doable) and even if the code is a mess. It's their responsibility as developers to not take away people's freedom and to not put themselves in a position of power over users. They can use a Copyleft license if they don't want they code to be used by proprietary competition.
And honestly, nothing closed source that Steam does is really novel enough to warrant being open source. The value of Steam comes from its ecosystem and playerbase, as well as the backing of Valve themselves. That’s not something that an open source Steam server or client would allow people to compete with.
It's not about innovation, it's about people being able to control the software that runs on their computers. The rest doesn't matter.
I would like them to release an open source command line tool for downloading, launching and DRM-validating-ing games though. That seems reasonable for people who don’t want to run the full client and want something like Heroic or Lutris to be able to hook into.
They could release the code to a lot of things. For example their proprietary Steam SDK library. Currently games that are libre software have to use this proprietary library to use Steam's features. DRM is unethical too and if Steam was Free Software, people would probably just remove it (kinda like crackers already do) or at least make it less annoying. This would only affect Valve's DRM and modern games often have multiple forms of DRM, but it would be an improvement still.