[enahs-sf]

I wonder how route profitability comes into play with those airlines running older planes (old 737's, etc) that may break more frequently.

AOG (airplane on ground) is essentially a broken plane stuck at a gate getting fined massively by the airport because it is blocking a gate. In this situation, usually price elasticity of demand goes out the window with respect to airplane parts because what you'll pay in markup pales in comparison to what you'll pay in fines.

Is it more profitable to get new planes or run older ones into the ground? The answer to this question is beyond my depth.

Source: my first job out of school was SQL jockey for an airplane parts distributor running reports for the guys trying to predict which parts would be needed. The company had 4 guys and two vans on standby 24/7 to run parts to the airport. This was a very profitable business segment.

[rtkwe]

Broken at a gate should be extremely rare. In the outside case a plane can't move under it's own power there's still tons of tugs available to move the plane around.