They’d need to improve desktop Linux
a lot to threaten Apple. It’s far more tolerable on their relatively tightly-controlled and stable hardware platforms than it is elsewhere, but it’s still a features-weak jankfest compared to macOS. I mean user facing features relevant to any desktop user, not “docker is native on it” or other developer-only stuff—and for the record Linux was my main desktop OS for about a decade, so I’m far from unfamiliar with it, and I do own a Steam Deck and have used it extensively in desktop mode (and in console-alike mode).
I’d love someone to actually compete with Apple at the specific kind of thing they do, but I don’t see it in the cards for Valve. Too much distance, with things they don’t have to solve to hit other (apparent) targets of theirs.
As for Microsoft, what is Valve threatening? Home no-business-use-case (mostly gaming and maybe light web browsing) PC owners, and I suppose x-box? The former has got to be negligible at this point, and the latter… I guess maybe, yeah, they could threaten that.
[edit] to soften this somewhat, I do love what Valve is doing and their micro-PC thing they’re releasing next year is likely going to be an instant purchase for me, provided supply issues don’t drive the price insanely high or otherwise mess with the release. I happen to be in the exact niche of people who are thrilled to have a good low-tinkering option that lets me ditch my last Windows machine, so this stuff’s my jam.