Ill put it this way one of the reasons GR was so quickly accepted by the scientific community was because it explained Newtonian mechanics. When a new theory replaces an older theory it usually does so by explaining the limitations of the older theory. It is extremely rare to find an instance where a newer scientific theory completely refuted the older theory, as opposed to merely explaining it to a deeper more precise level.
I am sorry, but i think that the last sentence more or less just states what i said right from the beginning. A new theory needs to provide better or more explanations than the last one. So i see no real disagreement here.
I only was confused when you stated that a new theory would have to explain the "success" of the old one. Thats what caught my eye. I see no reason nor any sense in discussing or explaining why a theory that has been or will be replaced by the new one had success. Perhaps i am just misunderstanding what you originally meant.
I do disagree however that gravitation was so quickly accepted because it explained newton. At least it shouldnt have ! It should have been so easily accepted because it explained gravity and observations that newtons ideas could not. Perhaps we are merely arguing about the wording here.
With GR i actually meant general relativity. In my understanding general relativity almost contradicts QM while special relativity could potentially be unified with QM.
But it has been a while since i spent time on these topics so I might be wrong.
I have read the rest of your post and do not see any point of disagreement there between us.