"Steam will probably still outsell everything else combined by 100x"
Yeah, it's a monopoly.
That's not a complaint. It's not a value judgement. People think the word is automatically negative or criminal, because of how often that market power gets abused - but it is just the label for having that market power. Valve is not a trust. Valve does not do any anti-competitive practices. (Their 30% cut is obscene, but it's the same obscenity demanded by other monopoly storefronts.) Nonetheless, company after company keeps saying:
This store is the only store that really matters.
If you're not on this store, you're probably fucked.
We have a word for that.