Well... If we will, why not? But the thing is that in 20-30 years we won't be able to invent any new writing systems that UTF-8 won't cover. Single-byte encodings were doomed because of their single-byteness. The same awaits two-byte encodings like UCS-2 (aka UTF-16BE) - we already have extended code points for something that glamour hipsters call "emoji". Variable-byte encoding will never become obsolete.
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.
For example, in a hypothetical alien language, a hypothetical character "rjou" would have a code 0x2300740457 (all the previous codes are exhausted). We can't express this with a single code, so actually we split it into 2-byte parts and write "#" (0x0023), joiner, "t" (0x0074), joiner and "ї" (cyrillic letter yi, 0x0457). As we have a joiner between these codes, we know that we must interpret and display them not as a "#tї" sequence but as a single alien "rjou" character. I think you get the idea.
UTF-16 can handle stuff above U+FFFF just fine, it encodes that with surrogate pairs. Are you thinking about UCS-2?
The 21-bit limit for Unicode comes from the limits of UTF-16's surrogate pairs.