If we buy games from devs so they can afford to switch engines, won't that mean we'll end up with games that won't get updated because the Unity codebase will become obsolete when they switch?

Or is the thinking that devs need to swear that existing licences will work with the game once it's ported to some new engine?

I'd support existing Unity-based games if the devs declared they will port, and that my license will still work once ported. If the particular devs aren't going to leave Unity, I'm not sure I'd be happy to keep supporting them, because that will keep Unity in business, and I think an example needs to be made.

Also, I understand switching platforms will be horribly work intensive, but it's not like it's starting from scratch. If the code can't be ported directly, the logic still can (providing the new platform supports the necessary functionality), and the assets also exist.