I think in the case of most games it's the long runtime/development time that actually low key ruins their development. The longer the time stretches the harder it becomes to manage the constant fan-requests, own ideas and storylines. It's like with a lot of AAA titles that start with a clear vision, but it becomes muddier and muddier over time thanks to content bloat, changing designers and directors (insert changing patrons and requests here), diluted by personal side-plots and what you end up with is a game that looses it's core vision and overstays it's welcome. At that point a lot of the developers themselves actually get sick of developing for it or have highly fluctuating motivations that partially clash with fan requests, and then you have a recipe for a game that dies a "slow death". Which is why IMO a game should be at least 1/4 or 1/3 developed with it's first version/demo, then clearly plan it's scope and development time, scrap side-storylines if they get too long and convoluted and have a decent system of scrapping gameplay/sytem ideas that don't make it beyond the conceptional stage. It keeps the game "clean". Experimentation with systems should be a thing in the first half of the development stage, as fun as it is, and later on you should have a clear pipeline that you follow and minimum requirements for content delivered (at least X portion of originally planned main story content in Y timeframe (months normally)). And once the game is at the end of it's live-development cycle there should be a way to draw it to a natural conclusion. Any plot-threats that are side-plots, popular side-characters, storylines hinted at can be deal with in sequels. You don't need to finish the entire story in one game, you can always close a big chapter and put a bow on a certain part of a storyline and then make a clear cut, look at what you are working with and then decide if you want to continue in the same universe / with the same characters or if you want to explore a completely different game/genre/character and come back to it later. That keeps things fresh, both for you AND your fanbase and it ensures you haven't been working on the same game for nearly a decade without ever having "finished" it, making both you and your (former or current) fans jaded as all hell.