How would you feel about an email service that didn't allow you to forward all your email to another account?
Yet we post our writing and photos to sites that don't provide the ability to redirect to the new location of our work if we think it's time to move.
So I'd add another requirement. It's an either/or.
Either you let me point my own domain to my content on your service, or
You provide the ability to redirect from my section of your site to whatever site I choose, for the indefinite future.
I suspect a vendor who wanted future-safe certification would likely go for #1, it's a lot cheaper. The "indefinite future" part in #2 means they'd probably have to buy insurance of some kind (a product that doesn't yet exist, but should).
PS: See the Aug 17 piece about criteria for future-safety. There's a lot more to this.