Others have already pointed out to you that it does not fund “luxury” cars. If somebody chooses more than a basic car (i.e. presumably spending what they would have had to spend on a car if they were not disabled in addition to their PIP, so that they can get a car that is suitable for them which they presumably couldn’t fund without the PIP element also otherwise they’d just buy the cheaper car themselves and choose to receive their PIP as cash instead) then what business is it of yours? Disabled people can spend their own earnings as they choose without your permission.
As has also been noted, the trading company of Motability makes a significant profit and repays some of this to the charity. It is therefore not costing the taxpayer more than the PIP that people would otherwise receive in cash; if anything disabled people are overpaying for the cars on less preferential rates than could normally be accessed by anyone via a standard leasing agreement (even when you account for Motability including insurance).
The reasons many disabled people choose to do this are numerous but include: simplicity (necessary for people with cognitive issues), high insurance costs for disabled people (that pesky levelling the playing field issue again), lack of access to finance on an individual basis (due to low incomes etc), or requiring specific adaptions that wouldn’t be possible in many second hand cars or allowed via a standard lease agreement.
If your concern is that the cost of PIP is too high then from a financial point of view Motability is irrelevant because, as I stated, if the Motability scheme was cancelled tomorrow the cost of PIP to the taxpayer would not change at all. It is simply a way to use economies of scale to ensure disabled people can participate in society including, in many cases, get to work, when otherwise they could not do so.
It also means that there is a larger supply of second hand cars in good condition with usually few miles on the clock that others can purchase, so has this side benefit for non-disabled people.
Given the scheme has zero impact on you as a taxpayer, perhaps you could explain precisely what your problem with it is?
If your issue is with the cost of PIP itself then Motability is irrelevant so it’s odd that you keep mentioning it. You keep claiming PIP should be means-tested yet I have explained to you earlier this morning that this would also have a negative economic effect (increasing processing costs, increasing the benefits bill, reducing tax revenue) which I note that you have - conveniently - ignored.
The two separate points you seem to have tried to make (your wish to scrap Motability and your desire to have PIP means-tested) would have no economic benefit and in fact increase taxpayer costs, so I am interested to hear what your motivation for this is if it is not simply just spite at the idea that these disabled people might have access to support schemes that you do not, because you are not disabled.