But instead they told everyone to just build it themselves... resulting in the current situation. They could have also solved the updating problem in the same way.
No big deal though, someone will make this build on other platforms if there's interest. It's all open source and I'm sure some 14 year olds on XDA are already racing to make it build on their phone from 1982. Then I'll probably flash it. Because that's apparently who I trust to write my phone ROM.
But my understanding is that they couldn't "just" do something like what Windows does, they don't get to boss the OEMs around in that capacity, some of the big ones effectively have their own Android forks (Samsung) or have already forked (Amazon), and if Google starts bossing them around they're just as likely to fully fork it as be brought into the fold.
They ABSOLUTELY do get to boss OEMs around in that capacity.
So no, they definitely don't get to boss the OEMs around. Unlike Microsoft with Windows they don't get to just say "you can't ship Windows^HAndroid anymore".
They do have a bit of hold over the OEMs in the form of it being a PITA to fork, access to Google's own apps etc. So I'm not saying they have no leverage, but it's a lot less than what Microsoft has, and definitely not enough to say "my way or the highway".
As a result, the device is supported only by the kernel they provide, rather than by generic Linux. And their kernel never gets updates.
ARM do not really have this. Unless you know exactly what device is on what address range etc, you risk sending the wrong signals and fill its firmware with garbage or something.
This means you can't really cook up a generic kernel package and apply it across the product range as you can on PC.
Your statement is true, however, that someone knowledgeable of the actual hardware has to create the correct FDT. This is slowly getting better and easier.
The more intractable problem, as others observed, is that ARM hardware vendors tend to throw together a custom kernel for a given ARM processor and board and then abandon it.