I think getting to a place that's new from a place that's familiar is part of the fun. ST09 was this, a transition to a new place and time. I think that an awesome mid-story could be a dark and gritty serial and a series would be in a future distant to us, maybe a generation who looks at Picard and Janeway like they saw Kirk and Sulu, badass. Trek never RELIED on the predecessors as much as it just barely mentioned them, albiet it did consistently have a formulaic feel.
You're not answering his question.
What point would backtracking to the old timeline serve the franchise?
What possible story reason would there be for doing so?
How would this, in any way, shape, or form, entice new fans to come in?
How would this not be anything other than sticking
Trek right back into the niche ghetto that almost killed it and catering almost exclusively to the shrinking core fanbase of diehards?
I don't think these are unfair questions that are being asked.
The point of "backtracking" to the old timeline is moot. It's not a backtrack. It's not a "whoops, it's broken again, lets fix it". The point with a new series/mini-series would see us in a future of what is now, a future which is really bad. Like those Borg from Enterprise received their subspace message before Archer destroyed them and went into overdrive getting ready for an onslaught. Bear in mind those Borg were from a future that doesn't exist anymore. So the point of going back to the original timeline certainly isn't for the sake of doing so. It's a plot device for the characters in our scary new trek to find hope in making things better for the galaxy.
The story I'm pitching to you that would make this happen is simple. Abrams-Verse. 25th century. Borg vs. Dominion. That's it. You're surviving. Maybe the lead character is a smuggler. Maybe he's a drunk. Maybe he's the last relic of the old Federation or, better yet, Section 31. Maybe he's got a small crew, which includes, for brute force, a Nausicaan. Who knows? All he knows is what he "believes", and that's some mysterious El-Aurian who convinces him, in a very vague way, that this reality isn't the best for either of them. You can build off this, but in this possible future, the war between the Dominion and the Borg would be a great catalyst for someone to get out of that world and into another one.
It would bring new fans in because it wouldn't be Trek like we know. It would be different. I'd like to say that potential writers wouldn't be afraid to kill off main characters or show really bad things. But at the same time, like in the new BattleStar, there needs to be hope, so, in the end, you get to "Earth" and the sun comes up, and life goes on.
And while a return to the prime timeline would run the risk of sticking Star Trek back into a niche, I would challenge that if you can't write a story in that Universe that isn't new, exciting, or something as almost recognizable as Star Trek, then you're not doing a good job as a writer. Pretend for a moment that a new Star Trek didn't involve Rick Berman and anything is possible.