The government has a monopoly on regular mail though (at least in the US). Should email be treated as a utility?

That would be one way to do it but the problem isn't confined to email. Losing an online identity means losing, in practical terms, access to a wide range of government services and regulatorily required filings.

Total bans are convenient for the big platform providers, but they are big enough to be regulated such that people don't lose access to everything, and that there are timely and responsive ways to challenge their decisions.

If the industry can't do that on its own, it's a job for the regulators.

Something like that. I think DNS registry system works pretty well under ICANN supervision: user isn't tied to a specific registrar.