Looking at it from a very simplistic viewpoint, say you have washing, ironing and drying to do. Do you stand looking at the washing machine until the cycle is finished? No. You put on some washing, hang the previous load out and then do some ironing.
And to get back to dogs: people say why spend money on fixing the problems of an elderly dog when that money could maybe spent on several younger dogs. It is choice. Just because an elderly dog is being looked after, doesn't mean younger dogs are going to miss out. Other people will look after the younger dogs.