They are the same character, though. They do not use the same glyph in different language contexts, but Unicode is a character encoding, not a font standard.

They're not. Readers native in one version can't read the other, and there are more than handful that got duplicated in multiple forms, so they're just not same, just similar.

You know, obvious presumption underlying Han Unification is that CJK languages must have a continuous dialect continuums, like villagers living in the middle of East China Sea between Shanghai and Nagasaki and Gwangju would speak half-Chinese-Japanese-Korean, and technical distinction only exist because of rivalry or something.

Alas, people don't really erect a house on the surface of an ocean, and CJK languages are each complete isolates with no known shared ancestries, so "it's gotta be all the same" thinking really don't work.

I know it's not very intuitive to think that Chinese and Japanese has ZERO syntactic similarity or mutual intelligibility despite relatively tiny mental shares they occupy, but it's just how things are.

Yes, but the same is true for overlapping characters in Cyrillic and Latin. A and А are the same glyph, so are т,к,і and t,k,i and you can even see the difference between some of those.

So when are we getting UniPhoenician?

This is a bullshit argument that never gets applied to any other live language. The characters are different, people who actually use them in daily life recognise them as conveying different things. If a thumbs up with a different skin tone is a different character then a different pattern of lines is definitely a different character.

It doesn't matter to me what bullshit semantics theoretical excuse there is, for practical purposes it means that UTF-8 is insufficient for displaying any human language, especially if you want chinese and japanese in the same document/context without switching fonts (like, say, a website)