One more about euthanasia - now I'm not sure...?

Would you put your dog down if without a doubt whatever he has IS terminal but they can't pinpoint exactly what it is? I'm second guessing our decision. All the vets we have gone to are convinced its one of 3 terminal illnesses/afflictions but all agree that the only way we could determine what it was is a necropsy (autopsy). Obviously we're not going to do that - if he's already going to be dead, who cares what it was. I'm only interested in the possibility of us figuring this out...