I have no doubt that if this were legislated then ISPs could do it - at no explicit cost to the former customer. It would be inefficient in that all email would be “routed” via the old ISP, taking longer to get to its destination and using up more bandwidth doing so.
There would be problems though …
What if eventually you forget your username and password with the first ISP that you had 25 years ago, and whose domain you are still using even though you long ceased to have a relationship with them, and so can no longer update the forwarding? (By then you have moved ISP seven times and you have to update the first ISP each time you move.)
What if the first ISP goes out of business? Or the domain simply goes away due to corporate restructure?
What if you eventually create a forwarding loop?
Most people who have looked at this problem decide that your best option is to take the one-off pain of replacing your ISP email address with an email address that is independent of your ISP. You can take as long as you need to do that and then leave your current ISP.
Analogies with global roaming or mobile number portability aren’t really relevant. The mobile phone network was designed to be able to do this. Email was not.
