Whereas if x86 systems have support on the mainline kernel, future kernel upgrades will be supported.
Any reason there couldn't be a "canonical" ARM64 standard for SoC?
The problem is a lot of ARM SoCs have tightly integrated, custom hardware which requires modified or new drivers, and the tweaks needed to use them are often either very dirty and won't be accepted into mainline Linux without basically redoing them, or are occasionally just hooks for proprietary userland blobs to interface with, and are effectively obfuscated, satisfying the "letter of the law" for GPL but no more.
There are some devices like this on x86 too, FWIW -- Google's Pixelbook, as an example, has a few devices that effectively need a custom fork of Linux to get the audio device to function correctly, because it's driven over i2c (IIRC) and needs special blobs uploaded and an out-of-tree driver to function.
ARM SoCs could be more "PC like" but it'd be more expensive, which as far as I can tell is a big reason it hasn't happened. No real incentive as people don't seem to care if their OS goes out of date in 4 years.
Luckily it seems like Framework do have this well under control (somewhat quietly :)