Technically, a superset would have to somehow Schrödinger's cat around \ in latin1 and ¥ in Shift-JIS being the same codepoint. Unicode just took it upon thems

Technically, a superset would have to somehow Schrödinger's cat around \ in latin1 and ¥ in Shift-JIS being the same codepoint. Unicode just took it upon themselves to reliably round-trip legacy text... thus the precomposed forms. Most of the other complexity and technical debt is in the writing systems themselves.