I don't disagree with what you're saying. But, riddle me this...
Most airplanes are 3x3 (three seats together, an aisle, then 3 seats together). Most (all) passengers would prefer a window or aisle over a middle, yes? So if it's that important to people, they purchase those seats. So (just to keep the math easy), lets say there are 20 rows of seats. Each row has 2 windows, 2 aisles, and 2 middle seats. With me so far? This is extreme, but let's say the first 80 people purchase all the windows and aisles (by purchasing them). So only middle seats are left. A mom with a 5 year old purchases two seats. Now, Rows 1-20, Seats A, C, D, E. Who do you move from their purchased seat to a middle seat?