This probably isn't the first FT thread that addresses POS in same way.
Airlines should offer the option to buy a second seat next to yours for the same price. Make it an option you can add during checkout, and even include a diagram illustrating how many inches is available for a particular plane's seating configuration. If you occupy two seats, you should pay for two seats or upgrade to a higher cabin. This option could also be helpful to people with guitars or cellos, or people who simply want a 2nd free seat next to them.
Pax ought to be able to fit between both armrests fully down -- in fact, we should have a model seat to see whether pax can fit, in the same way we have little metal boxes to see whether carry-ons can fit. If the pax can't fit in the model seat at the airport, then said pax should have to pay for a second seat -- not the same price as he paid at time of booking but the cost of a new ticket just prior to departure. That'll incentivize POS to plan ahead.