Over the years, the itch they were scratching tends to get fully scratched, people move on from their needs to other things. What was enjoyable, becomes grunt

Over the years, the itch they were scratching tends to get fully scratched, people move on from their needs to other things. What was enjoyable, becomes grunt work, like making the full set of unit tests for features users want, but you don't actually use yourself. Whatever this handwavy 'other benefit' was, stops being beneficial. But they're still paying the opportunity cost, sitting down as the older, greyer, burnt out maintainer while others get the benefit from their work.