System partition is read only. Updates in modern android are not file based but block based (to allow dm-verity to work, since before and after each update every block's content is guaranteed). If you remount /system as read write even once, updates can no longer apply, since block diffs applied to a filesystem different than expected will almost certainly do bad things.

Any full uninstall of a system app would require remounting /system as rw.

What is your beef with "disabling"? Under the hood it is the same thing as uninstall, minus the file removal or the actual apk. App doesn't run, has no permissions, is never loaded/mapped into RAM.