I don’t know tbh. But a year is a long time. My only thinking is that if the hire car is of similar size, spec and mpg to the damaged one then you don’t have much of an argument to cancel and go for a new one.
Also what’s wrong with a four year old car? There argument might be that many mobility customers are extending leases and being happy to have 4 to 5 year old cars. You may say that you’ve paid for a new car and that’s what you want, but at some stage that car will be 3yo and I don’t see a big difference between 3 and 4 year old cars. Mind it is a hire car so god knows how it’s been treated in those 4 years.
So think about what hire car they gave you and what mobility car you had and that may answer it for you.