The forms 𝜙 and 𝜑 of "lowercase phi" to have different codepoints makes perfect sense to me. That doesn’t mean that upper-case variants of these can’t share a codepoint. As presented elsewhere in this thread, "X.toUpper().toLower()" doesn’t have to be "X". The same holds for "B → b" and "B → β" depending on the context. It’s just that the savings from such a unification would be far smaller due to the smaller sizes of the relevant alphabets.
OK, but you still have a problem because I want to use the same font for Greek and Russian. What if my font is CURSIVE?
Russian and Greek have different cursive forms. You might unify κ and к, but actually the cursive form of κ looks like a Roman u.
So really if this were to happen you'd have "Russian" fonts and "Greek" fonts. Kind of like how Japanese and Chinese have to use different fonts for their languages.
Here's Cyrillic lower case: пф Here's Greek lower case: πφ
in some fonts the pi would be rendered with a longer bar on top, but you just showed why it's a bad idea:
I would want to be able to discuss Greek in Russian on a forum, but this would not be possible because all the glyphs in lowercase would look Russian