Yeah, it would def need to be easy and cheap to retrofit. Almost like Segways built into the seat or something? Probably makes more sense to go after carry-on luggage. Getting 200 people to sit in assigned seats doesn't take that long, it's all the crap they bring with them.
The machinery to get the seats in/out will be heavy, mechanically complex. This eats into revenue. Might not even work with existing airplanes.
You'd need to have machinery in the boarding lounge - at every airport the plane visits.
You don't really need this everywhere - people can just get on/off in the legacy mode - but when you visit an airport that lacks the gear you're just hauling dead weight around.
Okay, so we design a plane from scratch, and retrofit the terminals in the US ... but then you've got a plane that can only be used in the US. And one that can't be resold to Elbonia Airways when Northwestern is done using it.