It's morally acceptable to kill a mosquito but killing a cat is morally unacceptable. It was morally acceptable for Epicurus to own slaves but it's morally unacceptable for us. I'm giving these examples to show that human morality is liquid, ever changing set of societal rules arbitrary agreed upon by majority of people at any given time.
I think these are good points. In an Epicurean context where virtue (morality) is keyed to the production of pleasure rather than anything absolute, I suspect you're right to think that "morally incorruptible" wouldn't do anything more than point the finger back to the ultimate questions such as:
Can they sustain their lives of pleasure indefinitely, and is there any "natural law" reason that would prevent them from doing so?
We observe here on earth that all things that come together ultimately are broken apart, but I see no reason why that must translate into a law of nature that cannot be - through natural means - indefinitely postponed in time. Certainly I don't know how to do that myself, and I doubt it will be discovered in my lifetime, but I see no reason to postulate that it will always remain technologically out of reach.
The earth and sun and moon etc don't have the intelligence to alter the facts of nature that were started when they first came into being, but can we say for sure that intelligent beings can't develop to the point of doing so? I would say that the odds of that happening over infinite space and infinite time are actually pretty good, and I'm intentionally understating what the odds probably are in reality.