Changing results years later would be more damaging to the sport than all the doping in the world.
I don't know about "damaging", but it would certainly be more futile as you so clearly pointed out. That is why ASO revoked Riis' victory when he first announced he had used EPO to achieve it, but then some weeks later restored it. I think they realized the same thing -- that everyone was on EPO that year.
It was kind of a joke when they revoked Floyd's victory and gave it to Pereiro. You didn't have to perform any blood tests to know he was doping. All you had to do was listen to what he said.
If he were clean he would have said, "I'm so glad that they are catching the cheaters. That makes it so that clean riders like myself have a chance to win."
But instead he said, "I don't want to take the victory under these conditions. I don't think they treated Floyd properly." But after a couple of months he changed his mind and was glad to take the Yellow Jersey and the 1 million Euro prize money. A$$hole.