You asked, I answered... I think given the barriers (which others have touched on, and MANY more) it's just not feasible to do this.
I too wince when I see plumbers vans with 'email@example.com' on the side of it, but trying to do 'email portability' is just reinforcing a short sighted and deeply flawed behaviour.
The best way to solve this for people that care is to do it themselves. There's a one time hit (with LOTS of ways to mitigate the impact) that will leave people with a far better outcome.
Cheers - N
Disclaimer - I work for Telecom. It's right there, to the left of the post, but sometimes I feel the need to point it out, lest I be accused of just being a shill, for 'the man' :-)
It is definitely feasible, and Telecom has been able to do this for years - i.e. an email only or very cheap dial-up account.
Orcon do it for free (or at least last tiem I checked).
Vodafone use to do a $5 per month email only account.
I am not talking about any DNS MX fancy tinkering; just a low or zero cost way for customers to retain an existing email address when they change ISP's.
The problem now is the lowest cost option maybe something like a $20 per month dial-up account or as more ISP's only sell in bundles - there is no option apart from staying put.