Because you're looking at it on the surface. You can easily, EASILY, go do a million different things in the TOS-era.
Well, of course. But you are always going to be restrained by what will happen later (again, a creative writer can easily operates under restrains, and working on Trek already comes with a lot of restraints. It's just
one thing more).
But do you know what you can do in a post-
Nemesis setting?
Easily, EASILY go do a million different things
And in the words of Rick Berman/Brannon Braga themselves (when originally asked to do a 26th century show after VOY), "What are we gonna do…warp 14? Have even tighter spandex?"
That's not one, not two, but three different writers and producers from different backgrounds thinking there's not much else to do after VOY. I think they're right.
Well yeah, Berman/Braga did what, 14 years of Trek without a break? Of course, in the end they are going to get to the limits of their creativity. That's when you should do a change of the guard. New writers are going to do their own thing, and the era it is set in is only tangiable of importance. Fuller seems to have a love for prequel settings (as was for example his Hannibal). It can work, because he's talented and has experience with it. Doesn't change the fact a prequel setting comes with many more difficulties than a sequel setting, and we will see the effects on at least in
some way with some of the writers.
That's just the faster warp they are talking about. It doesn't change anything from a storytelling perspective. Much like Hull Plating served the exact same story purpose as shields.
There are some basic Trek staples that will never change. How space combat works for example. It's going to be a very similar way, wether it's set 100 years before TOS, parallel to TOS, or 100 or 200 years after TOS. Much like movies will depict air combat always the same way, wether it's a WW2 movie or "Top Gun", even though realistically they should look a lot differently.
I, honestly, can't think of anything that would be more dull. There are just flat better venues to tell stories about the military and political fallout of a fallen empire.
This is were I have to 100% agree with you. A "dark times" story might be interesting, with rising tension and threats from the outside and within, where humanity and humanism prevails. But all the "Fall of the Federation"-types of ideas always pretty much start and end there, since it doesn't really change much from a storytelling perspective (it's still going to be the adventures of a crew on a starship), but piss off all Trekkies beholden to Roddenberrys ideals.