I think you underestimate humanity's aptitude at creating things that don't fit into well defined standards.
My (admittedly poorly stated) point wasn't that we shouldn't be moving everything over to UTF-8. I personally use it wherever possible just because it makes life easier. My point was that there are decades of things that use ASCII-US or another one of the overlapping but incompatible encodings because they were the RightThing™ to use at the time and there's no way we're going to get rid of everything non-UTF-8 any time soon.
In 20-30 years we'll be saying "Why isn't everything in FutureText-64, it should be the only encoding. Why does anything else even exist?", and it'll be because we're saying the same about UTF-8 now.
But my main point is another: eliminate all single-byter and fixed-byter zoo and leave one universal encoding. When (if ever) it's time to replace it, we'll do it all and at once, not having those crazy iconvs everywhere.