steve0113 & I have been on 4 - DC to Zurich, Zurich to Rome, & the reverse. All 4 were packed solid, & on the one back from Zurich, I was the one encroached on by a two-seater, who was also wearing long flowing robes that constantly trailed over me. Every time I moved, her robes would be tugged on & she would turn & glare at me.
Like Steve said a few posts back, they should just put a seat (with non-moveable armrests) at the checkin. It's set up like that for carryon bags now & no-one complains about it being unfair. If your bag doesn't fit in the box, then you check it in. If your seat doesn't fit in their seat, then you need an extra.
I wouldn't have a problem with that solution.