In a perfect world there would be enough content that you only ever need run everything once, never needing to run them again at a different difficulty level. Never need to farm or repeat anything and still be finding new stuff after 3 years but that just cant be the case.
There isn't infinite content, it isn't generated. How much fun would it be if it was?
Seeing as though repetition IS part of the game though, some variance in the quests themselves between runs, monster locations, trap box locations, types or even the spell lists of casters so that there's more unpredictability in them might go a long way to making the repetitions a little less taxing. Then there might not be such a pressing need for new content, people would be more content with what there was if there was more enjoyment in it.