I agree with everything you said about the cost.  However, large airlines do keep hot spares at a few locations.  Low turnaround time is great, but sometimes something major breaks and you need to be able to pick up the slack.  There are even companies who provide "generic" airplanes for this purpose.<p>PS: The incentives are somewhat skewed.  Customers who have already paid for a flight still need to get where they are going so they accept delays.