On the one hand it will be sad if Steam declines as it is overall a fantastic service for gamers. On the other, they definitely occupy a close-to-monopoly like position in the market so it might be good to have that opened up.

    Eventually the people that run it now will age and die, 
    and one day the vampires will move in to suck its corpse dry.
    May not be our problem but will be someone's problem.

There's gotta be a fourth verse here that completes it.

They're a storefront that charges ~30% and they're pretty dominant in their market.

They make enough money —they're practically printing it— to never need investment, and they don't seem to piss it away like your FAANGs.

Which is all to say, even down the line, why would they risk it all for more money? Might seem naive but private ownership allows them to maintain this course.

It's possible that the company could be converted into a worker cooperative, which wouldn't be much of a stretch given their current (largely hierarchyless) company structure. There is no reason it wouldn't work.