I think it's not just the eject button, but Porter's service automation model being designed to move elsewhere with equivalent moving parts underneath.

I wouldn't be surprised if as a result migrating from Porter to elsewhere without using the eject button would probably be easier than migrating from fly.io - a sensible architecture on fly is likely more different to a sensible architecture on a 'normal' cloud platform.

(I could be completely wrong about all of this and would want to see worked examples before I get more certain than "wouldn't be surprised")