I'm all for a Star Trek set in TOS or pre-TOS era.

Why? Because not every story has to be an epic 'save the universe' story. There are an infinite number of stories to tell in every time period of the Star Trek universe. The politics of the universe at large are merely a backdrop. It would be like saying you could only have one television series set in the 1960's since we had Mad Men.

Star Trek is about people, the technology level and timeline of the original series is compelling to the average TV viewer for a really simple reason - they don't have to know any of the canon. And since ALL Star Trek fans are familiar with Vulcans, Starfleet, Klingons, etc, there's really no reason to alienate new fans by tacking on another 5 Television series of politics and galactic war.

You could tell a great Star Trek show set during the time period of Enterprise from the view point of an Orion smuggling ship and it'd feel more like Firefly even if it was Star Trek.

TL;DR Star Trek is a big universe, the period of time a show is set in that universe doesn't limit the kind of stories you can tell. Familiar is better for the general public when it comes to genre television and Paramount just spent hundreds of millions of dollars introducing the TOS universe to people.