My job requires me to travel fairly regularly with a rental car and I've gotten to know the guy at Avis that I rent from. I asked him how they decide to turnover cars as they had a 2011 GT that I wanted (Avis and hertz stock v8 mustangs, camaros, vetts, etc. they are just in high volume areas) and he informed me it is resale value based. Once they reach a certain depreciation level goodbye. Makes sense and explains why they have the cheap four bangers for so long. When your purchase price starts low, it takes that much longer to drop to a certain threshold to sell.