Once there is a payment, it's a business relationship. You don't want to enter a business relationship with an alumni who graduated many years ago that could be anywhere in the world doing anything, unless you can handle that. (Donations are different though, for whatever reason. It actually wouldn't be a bad idea to tie email storage with donations.)
Professors, staff and registered students etc, on the other hand, is easier to deal with.
> It actually wouldn't be a bad idea to tie email storage with donations.
This could be done in perhaps the opposite direction: individual alumni donors tying donations to maintenance of email accounts for all alumni.
These email addresses, in some cases, must date back to the 90s or earlier. Cancelling them is a major, negative change to people who (like me!) who have come to rely on them.
Billing is hard. University IT pays for X amount of storage across their Google Workspace tenant. For edus, you get a 100TB pool and can buy more storage in 10TB increments. It's not metered per user either.
Professors, staff and registered students etc, on the other hand, is easier to deal with.