Its in interesting point about standard PIP. If you lose DLA or PIP then you cannot keep the car as Motability is a charity and it would contravene the rules. However if someone is transferred from High rate DLA to standard PIP perhaps it might work something like this.
An additional AP would have to be paid to cover the difference between Enhanced and Standard PIP for the remainder of the lease. After that you would have the option to buy or return the car at the end of 3 years, no extensions. The only complication is where someone is awarded standard PIP and appeals. They, as I understand it would continue to receive higher rate DLA until an appeal decides their fate as would Motability, but Motability are clear that once you lose your allowance then after 4 weeks the car goes back. Of course the appeal may remove PIP totally and could take a year or longer which could compromise. A complicated state of affairs which needs clarification as soon as possible.