I don't see a massive difference between a Gmail account and a phone number. One is typically provided by a company, in a competitive marketplace, with rivals available. That one is regulated and has to adhere to portability which lets you continue to make and receive calls using that number indefinitely, without any further payment.
The other is provided by a large company, again in a (semi) competitive marketplace, but is totally unregulated and can kick you out at any time. They don't let you migrate your email address, and you can't send or receive mail to/from it if the company throws you into a Kafka loop.
It strikes me the anomaly is that email service isn't forced to be portable and delivered as a baseline utility, with "send on behalf of" and "forward without looking at or storing" as non commercial, non charged functionality that must be offered indefinitely, so a user can port their mail if they move away from a provider.
When looked at this way, I don't see the difference between mobile service and email - mobile providers don't enjoy a monopoly, but are regulated and need to deliver portability free of charge.