I believe if you do the maths then 5 year leases do not benefit Motability, with increased servicing/repair/MOT costs, a depreciating asset and the value of VAT free purchases being further eroded. Thus if 5 year leases were the norm I would expect AP´s to go up, not down.
Keep in mind that the biggest cost with a car is the depreciation in the first few years. Years 4 and 5 are far less. Also, customers are paying £6,500 in sacrificed benefits over years 4 and 5. Servicing, repair and MOT costs, plus tyres will be well under £1k for most scheme cars. The biggest saving for Motability though is the capital saving of only buying a new car every 5 years rather then every 3 years and only taking the early years depreciation every 5 years. I would bet a sizeable sum of money that lease extensions are a big winner for Motability, hence why the current situation suits them very nicely.