Here's another thing worth considering of how airlines order their seats. If the choice is 9-abreast of 34" or 8-abreast of 31", comfort level may be similar for the passengers, but the airline can put in more seats in the 9x 34" configuration than the 8x 31" one, by just simple maths.

Also, it's much easier to reconfigurate the aircrafts by adjusting pitch. With a 9x 34" plane, if future economics favors fewer seats at higher fare, then just remove some rows and move the seats further apart. Or if economics favor more seats at bottom fare, the squeeze them into 31" by adding rows. Meaning you can do it both ways.

But if you start with 8x 31", and economics require more seats at lower fare, you can't really squeeze the rows closer together. So, you lose flexibility.