I’m not concerned about this at all. When I sell, I can take out my 10-12 Kasa and Z-Wave switches so that the new owner can do what they want and not feel constrained by my past choices. Or I can leave them in and the new owner can still do what they want, which might mean installing their preferred technology (maybe they prefer ZigBee) or renovating the entire place from top to bottom.
This isn’t to say that you should do whatever you want without a care for the future, but that you shouldn’t put constraints on yourself based on assumptions of what the future owners will want to do. Besides, it’s relatively easy to replace lightswitches when the time comes. Knocking down a wall or putting in wall-to-wall shag carpet would be a bigger deal. 
This is a good point. When a prospective buyer tours your house, they might be very impressed by all of the openHAB goodness you’ve built into it and see that as a selling feature. So you’ll either need to leave behind your OH system (and they’ll have to learn it themselves) or stipulate that the home automation isn’t part of the deal to avoid having a very annoyed buyer.
This isn’t an openHAB-specific problem, as it’d be the same with any other automation system. I’m hopeful that the Connected Home over IP project will make a difference. Until then, it’s going to be a crapshoot whether you’re buying or selling, and any good real-estate agent would tell you to be more concerned about the roof than the lightswitches.
I’d forgotten about this option, as I don’t use the Echo Control Binding. There’s also this method: