It is interesting that, as a guess, we waste an average of ~5% of storage capacity for text (12.5% of Unicode's first byte, but many languages regularly use higher bytes of course).

I don't fault the creators of ASCII - those control characters were probably needed at the time. The fault is ours for not moving on from the legacy technology. I think some non-ASCII/Unicode encodings did reuse the control character bytes. Why didn't Unicode implement that? I assume they were trying to be be compatible with some existing encodings, but couldn't they have chosen the encodings that made use of the control character code points?

If Unicode were to change it now (probably not happening, but imagine ...), what would they do with those 32 code points? We couldn't move other common characters over to them - those already have well-known, heavily used code points in Unicode and also iirc Unicode promises backward compability with prior versions.

There still are scripts and glyphs not in Unicode, but those are mostly quite rare and effectively would continue to waste the space. Is there some set of characters that would be used and be a good fit? Duplicate the most commonly used codepoints above 8 bits, as a form of compression? Duplicate combining characters? Have a contest? Make it a private area - I imagine we could do that anyway, because I doubt most systems interpret those bytes now.

Also, how much old data, which legitimately uses the ASCII control characters, would become unreadable?