For one, rats are kept more as food than pets, and as such, vets don't get too many rats. I'd expect the vet to at most prescribe the rat some painkillers while he dies.
But life expectancy is more important.
Rats only live a few years. Your rat was almost two years old, and it was his time to go.
Sure, take a rat to the vet if you want, but if he's just old and dying, let him die. To use the cats and dogs analogy, I've known some people with really old cats. They get so tired and frail you can't hardly pick them up without feeling like you're going to break them. If that cat's going to die, let him die.