There's no reason for a complicated legal structure to ensure mail forwarding (which just begs for abuse). Users already have the power. Use it.
I agree though that there should be legal resort to not losing your email address.
If you think there's real demand for email portability, go into business! A service that lets you buy a domain and route to different email providers is well within the abilities of a single programmer. Make the UI idiot proof and charge a few bucks more than the registration fee.
All people sending email will be required by law to first consult the porting server to determine where to send email. It could be as simple as an http endpoint. Total development costs and running costs could be pretty low - low enough that the government can argue it is more than paid for from increased tax revenue from mail providers.
In the event that an ISP or digital services provider wishes to cease providing email service, they could be required to provide minimal access to enable updated provision of either a forwarding email address (which the provider would forward mail to), or an MX to redirect the sender to use. This would avoid the need for a big central "porting server", and retain the simplicity of doing a DNS lookup for one or more current MX records.
Given spam challenges, the former is likely unattractive (who wants to use their own IP ranges to relay potential spam to former users?), but the latter could likely work, or be made to work.
I realise the MX approach doesn't quite work, and you'd likely need to relay the email, but this is more akin to how the phone number porting system works - you ask the number block owner on each call, and they can either accept the call, or point you towards the correct destination network.
This topic is of indirect relevance and interest to the UK telecoms regulator [1], since many households rely on an email address from their ISP, which could become a barrier to switching, or result in long-term extractive pricing from users who have no real choice other than to pay a former ISP over the odds for email service to retain an old address.
[1] https://www.ofcom.org.uk/about-ofcom/latest/features-and-new...
In your example, anyone who wants to legally run an email hosting business.
What could possibly go wrong?
As for not allowing people to save their data before shutting down, I agreee. That's a consumer rights issue though, and not a technical one.
In essence, in the regulated telecoms space, the number block owner retains responsibility for routing to the new network after a number port.
For full porting, there's a long road ahead, starting with regulating domain names, which email addresses depend on. However, as long as the domain name _de facto_ remains in the possession of the original provider, this is technically possible, already.
Email address porting might even be possible without a persistent domain/DNS entry. After phone numbers get ported, the _sender_ (of sms, or a call) caches the new routing, and the original carrier doesn't know the ported phone number is getting a call in the new carrier's network. I'd be interested in discussing this sort of thing for well established internet services (such a central authority already exists, ICANN, but perhaps legislators can come up with a non-US authority for emails)