Was already bad enough that instead of bytes, we have to worry about code points. Now even that isn’t enough?
It would have been expensive, but all characters should have been fixed size 64bit values.
You're making the same mistake that numerous people made before you: thinking that it's as simple as using arrays of large enough numbers. First they thought that two bytes per symbol would be enough, then four. Spoiler alert: it wasn't. And eight won't work either.
Also, literally nobody wants to use 64 bit values to encode ASCII values. Even in our world of insanely large storage that would be breathtakingly wasteful.
It would have been a non-starter, and then we'd all be dealing with Shift-JIS, BIG5, and FSM knows how many different codepages to this day. UTF-8 is about as elegant as it gets, though Java and JS still managed to fuck that up too (they both encode every codepoint outside the BMP as surrogate pairs in UTF-8)
I can’t comment on Java, but JS I know reasonably well and I can’t think of any place it uses CESU-8.