The world is moving on, and while historic systems are beautiful (I still have a 2.11 BSD emulator running - or rather runnable - somewhere), at some point you need to weight the breakage for legacy users against the cost of maintenance of the compatibility.
Indeed, POSIX is still mandating that filenames are arbitrary byte sequences. But it is just becoming impractical, and in the end it's up to whoever has the motivation to have it working to keep it working, and if there's not enough people with this motivation it's just going to inevitably rot.
It's likely that 10 years from now, anything non-Unicode will be completely broken on modern (desktop, at least) systems and perhaps Linux even gets an opt-in mount option for enforcing filenames to be utf-8-compatible (which may change to opt-out another 10 years on, just as POSIX is going to evolve too in this regard).
Yes, it's a pity and I likely still have some ISO-8859-2 files from 1999 on my filesystem. But I think it's unreasonable for anyone to waste time with that support. And I wouldn't advise anyone wasting extra 20 hours of your developer life on building things around ncurses instead of a more direct approach - build a cool feature in that time instead!