But yeah I suppose you would need support for all the other foreign-language encodings that came in between -- UCS-2 for example.
But basically nobody does that. Glib (which drives all GTK apps' and various other apps file reading) doesn't support anything other than UTF8 filenames. At that point I'd consider the "migration" done and dusted.
In a UTF-8-path-only world, what I would do is have a mount option that says that the pathnames on disk are Latin-1 (so that \xff is mapped to U+00FF in UTF-8, which I'm too lazy to work its exact binary representation right now), and let the people doing archaeology on that write their own tools to remap the resulting mojibake pathnames into more readable ones. Not the cleanest solution, but there are ways to support non-UTF-8 disks even with UTF-8-only pathnames.
It is the bane of my existence, but many programs support all the Latin-1 and other file name encodings that are incompatible with UTF-8, so users expect _your_ programs to work too.
Now if you want me to actually _display_ them all correctly, tough turds...
You wish to find and delete them all, now that they've turned your home directory into a monstrosity.
Nobody cares that valid filenames are anything except the null byte and /. Tell me one valid usecase for a non-UTF8 filename.