Well Linux is usable as a desktop operating system. For IoT there are multiple open source project, Home Assistant, ESP Home, just to name a few.
The real problem are mobile operating system. Android phones are nowadays even more and more locked down, and using ROMs without Google Services is nearly impossible.
There's SailfishOS. It still uses Android kernel+drivers, but above that it's a "real" GNU/Linux system (glibc, systemd, bash, Qt, connman+ofono, zypp/packagekit, Gecko). It's not completely FOSS, but it is usable as a daily driver, and has been for at least 10 years (based on personal experience).
> and using ROMs without Google Services is nearly impossible.
Ever hear of GrapheneOS, CalyxOS, /e/ OS, LineageOS, divestOS? I have been using one of those for 2 years now, just like a "normal" Android. I bought my phone with it pre-installed, I didn't have to do anything.
Of course I can't use the apps that require the Google Services, but in my experience that's mostly just stuff like Google Maps (there are great alternatives) and YouTube (there are apps like NewPipe that work really well).
I think it's deeper than that; I think the problem is mobile devices. The OS has to somehow paper-over the fact that there's no mouse, and that everything has to be done with finger-stabs on a 3"x5" screen. That doesn't work with the traditional desktop widgets, so a variety of OS-level widgets and Javascripty plugins is layered on top. But (a) they're not consistent with one-another, and (b) they're not consistent with the desktop metaphor (which isn't going to go away).
Basically, I don't think a phone is suitable for user-input of any complexity. It's a device for selecting content that you then consume passively. It can't be used as a replacement for a desktop. "Mobile first" sounds all very well, but nearly all mobile-first projects have the desktop portion permanently stubbed.
> I think it's deeper than that; I think the problem is mobile devices.
I think it's deeper than that; I think the problem is app ecosystem. More and more apps that you need to use (e.g. the only way to perform 2FA with your bank) are dependent on Google/Apple services, use anti-root, anti-tampering and remote attestation techniques, making them impossible to run on free (libre) mobile OS alternatives.
The real problem are mobile operating system. Android phones are nowadays even more and more locked down, and using ROMs without Google Services is nearly impossible.