Once you are a franchise, you start to play things safe.
Franchises make money, and while you may be making X amount of money by doing the stories this way, the people behind you are happy. However, if you think you need to make changes, and you might end up maybe making X+Y amounts, but don't know for certain, they will dig their heels in. The franchise is all important. The franchise is making X amount regularly. The franchise must not change. No matter if it needs to grow and change.
They don't want you to change the franchise, even if it could become better, become it makes X amount regularly, and it might make less if you change it. Franchises are what has kept Star Trek stuck in their own personal time loop for ages. Build a sufficient body of work, and people will inevitably catalog it, analyze it and start to work out connections and contradictions. People will be going why didn't they use this invention here from that story back then. People will go if this is that, then that is also this, and wouldn't it be cool if.....?
Once you get into that looking back more than looking forward mindset, you're screwed. Eventually, and with all the good will and good ideas in the world, you start tying everything together in a ever-confusing, ever decreasing circle where nothing new can be introduced because you'd contradict this story from earlier, and spoil the franchise. The worst thing is : you can't get new people to try the story, because you have to explain too much. Or the story doesn't work without the knowledge of previous parts.
The only solution that appears to work, once you hit that critical mass, is the dreaded re-boot.
At this point, comic fans are throwing things at me. DC comics did the post-Crisis re-boot. Marvel comics are trying the back door Ultimate universe re-boot. We won't even mention the later Legion re-boot. People are still sore about these things after all these years. They lost years of their favourite stories that they grew up on, and want back. And Star Trek is sneakily trying this with Enterprise too.
The re-boot, in it's own way, is a beautifully simple idea. Start again from day one, knowing what worked and what didn't work the last time right. Start with the very basics and introduce slowly and carefully all the things that people loved the last time, and update them for the here and now. In the mid-80s Lex Luthor was re-invented as a corporate mogul, instead of the typical mad scientist inventor that he was for 50 years before that. That idea was so inspired, and resounded with the times so well, that it's stuck and has appeared in all formats since then : the comics, Lois and Clark, and Smallville. Ma and Pa Kent still being alive also dates from that era, and works too.
On a side note - anyone want to guess the earliest re-boot of fictional characters? The Hardy Boys and Nancy Drew, in the late 1950's. Every single Hardy Boy or Nancy Drew mystery that had been published between 1927 and then, were systematically re-written, updated and changed to better fit the times. Every single Hardy Boy that I grew up on in the 70's was a re-written book. Sales were falling, teens of the time were no longer buying the books and crisis point was reached. Harriet Adams, the owner of the franchise, and helpers took 18 years to do it, re-writing the plots, and on occasion gutting the originals. The names may have remained the same, but the insides were very different. Sound familiar, comic fans?
I guess what I'm trying to say is Change or Die applies to all things in the end. Stories and tales either remain locked in one time and place for the rest of their existence (Sherlock Holmes) or move on to new pastures green. The illusion of forward motion is sometimes all that matters, but that movement has to be seen. Enterprise is on that cusp now - actively mining it's own past, to ever-decreasing ratings. The Superman titles after 18, ever increasing in complexity, years are back to that cusp now.
I just hope it never happens to some of my favourites like Buffy and Angel. But I worry at times...