It's very sad to see them get like that. Our day foster dog is a large spaniel, and he can no longer climb into the car. He nearly drowned once, going after water birds in a flooded stream, because his back legs don't have the power to swim any more. His owner says the vet told her it was more like needing a hip replacement.
Even our border collie, at 13, can't jump into the boot of the car, only low jumps.
mustgrumble - I would ask the vet if there are any simple ways to help. I've heard of dogs with paralysis in the back legs being fitted with a sort of trolley, but I think that was due to a stroke.