There is some misunderstanding of the statistics here (wilful or not, I can’t be sure).
If 1 in 5 new cars are owned by Motability that has no relation to the number of cars on the road or the number of people who have cars.
Motability is an expensive, restrictive scheme for people who have to use it and they insist that cars are changed every three years unless you have very specific, rare circumstances. Those new cars are largely going to the same people over and over again, not all going to different people.
There are huge numbers of people (like me) who qualify for Motability but just can’t afford it so don’t use it. What I did do was buy one of those ex-Motability vehicles that was sold to a dealer when it came off lease at three years old. I did that because I have generous friends who helped with the cost.
There are a lot of vehicles bought by Motability because they constantly replace them, not because everyone can afford to have them. The rental payment is high, mileage and modifications restricted and the up front payment you have to come up with every time you are forced to change the car is huge.
Even vehicles like mine with complex adaptations and hydraulic ramps are replaced at the most every five years (mine was three when I got it) - Motability is a business as well as being registered as a charity and it makes money. It’s not a government body.
I can only echo what other posters have said about public transport - the last time I tried to take a train it was cancelled and the replacement bus service was a completely inaccessible coach. I wasn’t able to get to an important work meeting as a result. In that situation PIP covers the cost of getting a taxi that would take my wheelchair so I wasn’t late to work.
The mobility component of my allowance covers the cost of my vehicle so I can work and have a life - I’m lucky I can work from home but I do need to go in regularly too. Without it I couldn’t afford to work, and that isn’t going to help me or the economy.