Possibly you could look at Newton's example. Yes, his best scientific contributions happen when he was young, and they are what he is known for. But in his late 40's he gave that away. Perhaps as you say it got too hard.

Instead he took up a job at the mint. The job had some technical bits to it like designing coins that were harder to counterfeit, but is was mostly a managing people and society job. The coin clippers were literally debasing the currency to near worthlessness. Had they continued they would have been the internal cancer that took down the empire, as the state could no longer afford to pay for the ships and men needed to create and maintain it. Newton almost single handedly turned that around. It's not an exaggeration to say he saved the empire.

You say the older Newton could not do what the younger Newton did, and that's probably true. But it's also true the younger Newton could not have done what the older one did. He needed the experience and understanding of how men and society work that only decades of live could give him. So yes some abilities dim as you age. But other abilities grow stronger.