That is surely part of the consideration, but certainly not all. Some engineers at Valve (especially the head honcho Gabe Newell) are legit Linux people (Debian IIRC). They believe in it, and I love them for it

And it left them well-positioned for the steam deck, I wonder if they were thinking about that when they started steamOS, or if it is just an example of the natural advantage that openness gives you.

Anyway, agree—I wasn’t trying to belittle Valve’s motivations, just wanted to include a thought about why they seem to be happy serving both platforms.

I don't dispute your claims, but I remember very clearly that back then it seemed obvious that SteamOS was a response to the Microsoft Store and a fear that Microsoft would mandate that all software on Windows come from the Microsoft Store.

While that was obviously speculation, at least the dates match up (October 26, 2012 for Microsft Store launch and December 13, 2013 from SteamOS launch according to Wikipedia)

Agree based on my memory. I think the Microsoft store threat is what finally tipped the scale. It took it from "we kind of support linux because we like it" to "we support linux because it's important business insurance for us in case Microsoft goes Apple (or Xbox or whatever example you want) and monopolizes app distribution on Windows.