The proposal to keep middle seats free has no merit. Many people travel in groups (families, couples) who live together. They don't need to be separated, so it is a waste of capacity to cordon off centre seats. The solution may be a small programme incorporated into the booking engine where seats are allocated. this could be programmed to leave a pre-set gap between discrete parties. It would enable airlines to incentivise bookings of 2, 3 and 4 people together, as they would not require separating. Of course, it would rely on groups not living together to declare themselves separate.