The federated nature of e-mail is degrading. Most recent example: My kid just registered with CCCApply (the central website for California Community and City colleges).

It took a while. Finally after calling support I found out that the e-mail you get your confirmation code to doesn't work with most e-mail providers (they said custom e-mail domains hardly ever work, Yahoo e-mails never work, Outlook is iffy) so please register with a gmail address (and they offered to help me sign up for gmail if I didn't have it).

An older example: According to a Macy's CS rep, Macy's won't deliver their e-mails to domains registered with godaddy (the actual e-mail provider apparently doesn't matter, but in this case it was Microsoft). My mom has an account with them that she can't access, because she needs to receive an e-mail to login, and she needs to login to change which e-mail address is associated with it.

How so? Just because many people use mega-corp for email doesn't mean you need to. There are tons of alternative, small company providers or you can host your own. None of these require big-corp's support.

Because according to popular judgement, independent email hosting in the modern day is a very difficult ordeal.

Is that true? Is that false? How would one tell? One's own experience will be trivially handwaved away as an anecdote, people's experiences will be handwaved away as hearsay, and a claim of general consensus will be handwaved away by other claims that the person pointing it out is just living in a bubble. Principled thinking? Could be false, could be true, really - both would make sense. Doesn't sound very productive to discuss to me.

If nothing else, surely you can agree that despite what might be, what is is that email is incredibly centralized, right?

It is centralized (I wouldn't say "incredibly", but sufficiently) but because it is an open standard it doesn't matter. You can switch providers very easily with many options.