There's a problem with using email addresses as IDs which has nothing to do with Unicode: Is frobozz@example.com the same as frobozz@example.com? Don't bother comparing bytes: What's changed is the time, and who owns the frobozz@example.com email account. Names need to have some kind of stability over time to be useful, and need to have some mechanism whereby they can be changed gracefully if they need to be. Email addresses have neither of those properties.
Mailing addresses have the same problems to some extent, but at least most mail systems should have some concept of forwarding addresses by now. Email is never going to.
I don’t see this as a problem so long as you don’t treat the email as the permanent source of truth of the account identity and instead have it be the email you happen to have associated with the account at that time.
We have abstracted the notion of passwords as “a collection of authenticators defined by the user”, no reason your account can’t have a collection of identifiers as well.
I agree with you that "identity" has many attributes of which an email address is only one.
The problem is that in practice, control of the email address on record is sufficient to take over many modern accounts. Bank accounts likely shouldn't fall into this category, but most web sites don't have bank account level of security.
Hotmail created a hornet's nest of problems when they started recycling email addresses after 6 (or 18?) months of inactivity. It allows a quick "password reset", then the account is now owned by whomever controls the email address. Effectively, the identity is hijacked because control of the email address was most of the authentication mechanism. Queue the spam messages and fraud/phishing.
Also developing/managing a customer service tool which allows them to decipher these "takeover" events and ensure the person contacting them used to own the account is difficult and sometimes not possible.
Mailing addresses have the same problems to some extent, but at least most mail systems should have some concept of forwarding addresses by now. Email is never going to.