The thing is - your franchise's built in audience is the first and only guaranteed audience. So you have to go & expand from there.
MARVEL studios road to success was taking their heroes (Iron Man, Hulk, Captain America,...) - and being really, really faithful to the core identity (!) of them. But then shaving off decades of lore, trivia, controversies, rough edges, streamline the heck out them, and make accessable stories that anyone can understand without needing years of backstory knowledge.
Now they are struggling, because between dozens of movies, streaming shows, hundreds of characters, references, ... they are just not easily accessible anymore for newcomers, their stories don't feel fresh anymore, essentially the same problem their comics had before, before they did streamline everything for the movies.
OTOH DC & Fox had the problem that they tried to distance themselves from their characters - much more even then current Trek - Superman & Batman murdering criminals, the Fant4stic being gritty and unpleasant, X-Men afraid of wearing yellow - they just lost their core audience, and general audiences fizzled away slowly after that.
Back then DS9, VOY, ENT all went in with the premise of "more like TOS", "more action", "more modern", "different" - but then I would argue still all were easy too similar to TNG from the start, like the nth CSI spin-off, with diminishing returns over time, until the last one got cancelled before reaching 100 episodes.
By contrast the "modern" Trek period (well... 17 years since ST09...) - the main producers & directors regularly go directly to the press & say they never liked Trek and didn't understand the appeal of it, and now try to make a show for non-Trekkies and non-nerds.
And then no one else other than the hardcore nerd Trekkies show up, those get reasonable pissed, and the only chance to stumble upon these shows is if Netflix recommended it, because there is no positive word of mouth chatter among fans & friends at all. Hindsight is 20:20, but that this approach didn't work should not have been a surprise to anyone, to be honest.