On a practical matter, it seems like a bad idea to have codepoints that can take up to an arbitrary number of bytes - this just screams buffer overflow problems.

So in practicality, you’re going to want an arbitrary limit on this (the article suggests as much). But if you place a limit on it then you’ve got one implementation of the standard that can decode certain characters and another that can’t. Better to have one standard that puts a hard limit on the number of bytes and another standard that uses more bytes and so on.