We worry about companies that aren’t anywhere near as dominant as valve. Just because their interests align with ours today doesn’t mean they will tomorrow.

Valve is dominant because they treat users well. Is your argument here seriously "Yes, Valve is a better platform that treats you well, but you shouldn't use it because other people already do! You should use a platform that's not as good because competition!"

A competitor in any industry needs to do more than "exist" to be worth using. If Valve starts acting shitty I will stop using it, much like how I have stopped purchasing or playing Blizzard games.