I completely understand and sympathize with your point, however this:
"If you use a slightly niche OS (say, OpenBSD on ARM)"
Is rather hilarious - calling OpenBSD on ARM a 'slightly niche' OS in the context of 'operating systems people use to watch video on the web' is rather ambitious, don't you think? OpenBSD on ARM would have to increase it's user base in this arena by a couple orders of magnitude to elevate itself to 'slightly niche'....
I picked two relativity popular platforms, though niche together to focus on the danger. All the more so would something like the rust os have problems.
The bigger issue will be when these providers will start holding OS's hostage (well, you want to view half the web? You've got to have a locked bootloader and only I have the key. You want to index our sites? Google has permission (the unlocking key) but not you).
"If you use a slightly niche OS (say, OpenBSD on ARM)"
Is rather hilarious - calling OpenBSD on ARM a 'slightly niche' OS in the context of 'operating systems people use to watch video on the web' is rather ambitious, don't you think? OpenBSD on ARM would have to increase it's user base in this arena by a couple orders of magnitude to elevate itself to 'slightly niche'....