I have always wondered - what if the utf-8 space is filled up? Does it automatically promote to having a 5th byte? Is that part of the spec? Or are we then talking about utf-16?
So, it won't fill up during our lifetime I guess.
Imagine the code points we'll need to represent an alien culture :).
If we ever needed that many characters, yes the most obvious solution would be a fifth byte. The standard would need to be explicitly extended though.
But that would probably require having encountered literate extraterrestrial species to collect enough new alphabets to fill up all the available code points first. So seems like it would be a pretty cool problem to have.
So what would need to happen first would be that unicode decides they are going to include larger codepoints. Then UTF-8 would need to be extended to handle encoding them. (But I don't think that will happen.)
It seems like Unicode codepoints are less than 30% allocated, roughly. So there's 70% free space..
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Think of these three separate concepts to make it clear. We are effectively dealing with two translations - one from the abstract symbol to defined unicode code point. Then from that code point we use UTF-8 to encode it into bytes.
1. The glyph or symbol ("A")
2. The unicode code point for the symbol (U+0041 Latin Capital Letter A)
3. The utf-8 encoding of the code point, as bytes (0x41)