To be fair, quite a few Android phones have unlockable bootloaders. You go to the manufacturer's website, download a utility or request a code, and you can then also install whichever OS you want.

In my experience realistically even if you can unlock the bootloader, the hardware is still so proprietary that you have no hope of ever installing an alternative OS it because by the time you reverse engineer it all the phone is already 10 years old.

In my experience it usually goes just fine. I’m actually not totally sure what you’re talking about, what does the hardware have to do with it if you’ve got the bootloader unlocked? Assuming you’ve got something common like a Qualcomm SoC, it’s pretty straightforward to take codeaurora or whatever source and AOSP and 90% of the time have an almost fully functional custom image for your device.

Now granted I’ve rooted every phone I’ve had since the HTC Incredible, and I always check XDA forums before buying a new one to see if there’s a decent development community. And in recent years there has been a SHARP drop in the % of manufacturers that even allow their bootloaders to be unlocked - I don’t think the last like 6 samsung galaxy’s have ever been possible to do it. But there are still plenty, I’ve managed to find a kickass flagship model every year or 2 years and upgrade through at least 2 or 3 versions of android (I always buy last- or last-last-gen models used for <$200 so they’re typically a bit out of date already).

I couldn’t imagine any other way honestly. I absolutely love rooting & flashing custom roms, and it’s kept all my phones practically as young and snappy as the day they originally released...or better

There are different definitions of alternative OS. I'm guessing yours is a different build of Android with the original kernel blobs transplanted from the OEM rom. The other one is being able to run mainline linux or a custom OS from scratch.

If Linux didn't already exist it would never have been possible to create since all computing platforms are getting more and more locked down.

Is there any phone with full open source support? So you can compile Android and run it without any proprietary driver blobs.

Yes. The newest device is the Samsung S3 (2012) https://redmine.replicant.us/projects/replicant/wiki/GalaxyS...

You can also run mainline linux on the Nexus 7 2012 but you lose wifi support.

Most alternative OSs just use the patched kernel that shipped with the device.

That's a little bit sad state of affairs. But better than nothing, thanks! Latest version was released almost 3 years ago. I wonder if security being taken seriously? It seems that security holes are found more often than that.