Gaming

OP is comparing to tools that download and install games, but the Steam emulators you're thinking of don't do that; they only emulate a minimal set of runtime services that Steam games expect to be present in order to run.

They don't implement Steam's online features, like registering achievements and making cloud backups of save data, and don't have the extra features like input device remapping or video streaming. They are great for running games without network access, or for continuing to play games if Steam ever shuts down, but they're not really replacements for the Steam client.

I don't know whether Valve has opened the APIs for downloading games, registering achievements, etc. If they haven't, then a full replacement for Steam might still be technically possible, but it would require some reverse engineering and be vulnerable to breakage whenever Valve changes something on their end.