Just like any form of media franchise, a video game series can never consider itself truly safe. Even if you're currently the best selling title on a console and everyone is praising you, in just a few years you could be the laughing stock of the entire industry with just one medicore game. It's a shame, because it leads to some beloved franchises loosing their footing and never recovering.
This has led to some game franchises just completley dropping off the planet, such as the Duke Nukem series, despite being fan favourites back in the day. Other franchises may still continue, but only as a husk of their former selves, which is arguablly worse than just finishing all together.
However sometimes there are titles that seem to be doing well that just can't seem to catch a break. For some reason these franchises either can't seem to get the love and attention they deserve from developers or the games themselves don't manage to track well with audiences, despite being well developed and fun experiences. Either way, the franchise suffers as a whole.