You're right. For all of Google's other flaws, their hardware has been surprisingly good about this. The Chromebook write-protect screw is a good example of a thoughtful feature here.
However, Google's approach is so uncommon, that it's basically the exception that proves the rule. The vast majority of device manufacturers don't do anything like this, and if you were to pick a phone at random, you would probably expect to encounter some sort of issue flashing it. That's why we're still stuck with databases of "if you want to run a custom Android ROM on your phone / OpenWRT on your router / etc., pick from one of these known-good devices."
There are a lot of other devices (that aren't x86 PCs) which let you boot OSs wholly from third parties. A couple that come to mind are Canon Cameras[1], and the Remarkable Tablet[2]. A lot of routers, too, don't put up any barriers the installation of alternate firmware (some do), and even Apple Silicon Macs can be made to boot unsigned OSs (and are getting a Linux port). Not to mention the Raspberry Pi and its ilk.
I say this almost preemptively, because I really don't think it should obscure your core point: open devices are increasingly the exception, not the norm.
However, Google's approach is so uncommon, that it's basically the exception that proves the rule. The vast majority of device manufacturers don't do anything like this, and if you were to pick a phone at random, you would probably expect to encounter some sort of issue flashing it. That's why we're still stuck with databases of "if you want to run a custom Android ROM on your phone / OpenWRT on your router / etc., pick from one of these known-good devices."