People tend to have fewer friends and become less able as they get older, and even incidental interaction is still vastly better than none. If we had a way to make sure disabled people always have company and no one is left behind, I might agree to the idea of taking them off the busses and giving the (semi)private rides. But we shouldn't assume they all have friends to go see just because you always had such friends.

And then, spatial mobility is also very important. For example, I felt absolutely trapped by my relatively benign broken foot problem.