I think you will find many people booking extra seats. Particularly couples who are flying together who will book the middle seat just to have the row to themselves. This will still be cheaper than two first class tickets. and that's the problem. Airlines refuse to lower the price of first class seats, but are trying to create an artificial scarcity to drive people to pay the higher price. Airlines now usually have sizable wait lists, those cheap empty middle seats represent to them a lost sale on a first class ticket and a paying passenger they left behind. Not sure it holds logic if you think it through, but I don't think the airlines think many of their policies through.

I would definitely be happier with an empty middle seat, but again it's a) sitting on a seat belt which doesn't fit right, and b) arm room, which you still deal with from being too close to the wall or aisle. So, I would in the end still prefer one premium economy seat.

And as for the seat size, the typical coach class seat room is really undersized. I know some people complain about why they should have to pay any more for what they theoretically don't use, but I don't see why these people are owed a cheap price. If you would rather sacrifice to get a cheaper ticket, that is the price you pay. You risk not having enough room to be comfortable. That's the trade off.