So supporting Linux means they need to test on at least all currently maintained versions of maybe the top 20 or so distros

It absolutely does not mean that.

Pick a steam deck, support a steam deck, 3 major releases. If the SD runs on enterprise Linux that's a 10 year support window.

That's a perfectly viable plan - much like "releasing on x box" - and with an understandable market clearly delineated. Everything else can be "hey try, but don't call us" and we'd all still try.