Yeah that's a good point. In our case, all smart systems can work independently (via their own app or physical controls) or be integrated with other smart home platforms. It's true that if they would ever move out, the HA instance might be gone, making the house loose a bit of its brains when it comes automation. But the base to build on is there, still has value I guess. And the rack with Ethernet in all rooms has some value too.

Edit: The only "problematic" part would be the cameras. Not sure if Unifi cams can work with other platforms than Ubiquiti's. But in any case, the wiring is there (Ethernet) and the cameras can be replaced or a new owner could bring their own Unifi surveillance console (or we could sell it with it).