New story = new title/sequel. It's worked that way with books, films, and TV shows since the beginning of recorded human history, it's worked with video games during the entire 50 year history of video games, and it's for this reason that the same should continue to apply to video games until the end of time.
Except that in MK11 the Aftermath expansion broke that mold and so have other games that have DLC story campaigns / standalone DLC, and if done right can add a lot to the current series.
Hell, my second favorite game series (Dishonored) did this very thing with a DLC story mode that was pretty much a whole 2nd game (The Knife Of Dunwall & THe Brigmore Witches) and is probably my favorite part of that entire series lore because it further explored an awesome assassin by the name of Daud.
I never understood why is story expansions are seen as a no-no, but yet side stories and spin-offs are more welcomed.
Also, the Flash TV series started as a spin-off from Arrow, and there have also been examples of other TV shows doing spin-offs.
Not every new story = a new title / sequel.