Arguably the phones with open bootloader have the best Android experience out there (Nexus line), so I'm not sure if Android here is the best counterargument.
They have an open boot loader because Google sells them, and they have the best Android experience for the same reason. The open boot loader doesn't enable that experience.
In fact, I think that somewhat proves my point, it's no coincidence the hardware Google has the most influence over has the best experience.
And it's no coincidence they gave up on just influencing the phones and went as far as manufacturing a phone with the Pixel, all in chase of the perfect experience
Is UX a nightmare on desktop right now? Nearly all desktop/laptop computers can have their OS changed to different one. There is technically nothing stopping mobiles form working in the same way.
> There is technically nothing stopping mobiles form working in the same way.
What about power consumption issues? If you load up Windows on a Mac laptop the battery drain rate is doubled. That's a pretty big user experience issue IMHO.
I wouldn't call this a UX nightmare. This is certainly an issue. On the other hand, as in case of today's laptops, users themselves decide to switch the OS and it's their responsibility in this scenario.
Does anybody really care that much about operating systems any more? The big three on the desktop - Windows, macOS, and Linux - are all good enough.
What matters (and I think this has always been true) are the applications. When friends ask for my opinion on what they should get, I almost always recommend buying from Apple (gamers probably want Windows). The two big reasons are (1) they can (legally) run more software than anything else and (2) you can walk into a store for support that's actually pretty good.