If we're looking at sub-$400 computers, especially on ARM, it seems like we have to include the large segment of ChromeOS devices that only run Linux out of the box (or at all, generally).
Referring to Intel Chromebooks (i.e. laptops), that segment is now dwindling in size much as its predecessor (Intel Windows netbooks) did a few years ago. Most low-end ChromeOS devices now run on ARM. And Android is nipping at their heels.
Netbooks were originally Linux. MSFT created a special licensing class just to try to undercut it. It wasn't great, but because Windows and Microsoft licensing, it quickly took off. People realized Windows on netbooks sucks, thought that meant that netbooks sucked, and eventually netbooks died. Until, arguably, ChromeOS arrived.
Sure. And all of those devices run Linux. Some of them even run other Linux OSs decently; one of my daily drivers is an ARM Chromebook running postmarketos.
It is not trivial to get FOSS Linux onto a write-protected Intel Chromebook, compared to a Windows netbook of yore. It is harder still to get it onto an ARM Chromebook or Android tablet. PostmarketOS is a bit simpler (or at least better documented) but it is not a full Linux distro.
Installing a fully-fledged FOSS OS on low-end general-purpose computing hardware is getting harder. Certainly for the non-techies who have to be part of FOSS if it is to survive.
I think it's better and worse in slightly different ways. On the one hand, yeah a Chromebook won't let you touch the default OS without switching to developer mode, and won't let you install a new OS without disabling the write-protect screw or firmware option. On the other hand, every ChromeOS device allows you to do exactly that, and then you can run whatever you want and you should have at least some support for upstream Linux because ChromeOS upstreams their drivers. (I will happily agree that the Android device situation is awful.)
> PostmarketOS is a bit simpler (or at least better documented) but it is not a full Linux distro.
By what definition is PMOS not a "full" distro? It's Alpine plus some extra stuff, including device tweaks and out-of-the-box desktop environments.