-Make a TOS spinoff? Why not, but that would only work as a series
- Just keep making TOS movies with the current dudes until one of the films tanks?
My money is on them doing both, with the TV TOS cast waiting in the wings to take over the film franchise.
Look at the way
Iron Man has taken off. Ha ha to my so-called "LA insider" friend who told me the movie would tank because "nobody has heard of Iron Man."

(Damn I should call him now and gloat.) Well that's true, he's not a first-rank comic book character to the vast unwashed masses who don't really follow comics, but that just goes to show that if a lesser-known brand name like that can take off, with a bit of good management, Star Trek should have no problem doing the same.
A universe we already even know the future of. That's not a recipe for a long series of movies.
That won't be a problem, for two reasons:
1. When the new movie makes a whole lot of new fans, most won't know anything about the future events, or if they saw any of the series at one point, they won't recall, or if they recall (like me), it won't detract from their enjoyment. Again, look at
Iron Man. The movie is based on the character's origin, 45 some odd years ago, and the intervening decades have seen a mammoth amount of story created for the character. How many people standing in line for tickets now know or care anything about all that material?
2. Who says knowing the future makes a story boring? We know how the Civil War ended. Does that make
Gone with the Wind boring? We know how WWII ended. Does that make
Band of Brothers boring? LOTR was a popular movie series, and a large percentage of the viewers knew how the story turned out before they saw the first frame. Why didn't LOTR crash and burn at the box office?
Does Star Trek have a future as an ongoing Saga at all?
Yes, an indefinite future of enduring popularity and profitability. All it requires is something it hasn't had for a while: good management by people who understand its strengths and have the ambition to create something both respectful of the past, and exciting & new.
And as much as I love DS9, we won't see the TNG, DS9, VOY or ENT characters resurrected anytime soon. The focus for the foreseeable future will be going back to the 23rd C and building up that era, with the TOS characters at the core. Not that the 24th C is "played out" (the notion of an entire century worth of story in an entire galaxy being "played out" is bizarre beyond belief) but just because it's a smart strategy to build from strength, and Trek XI will in large part
create that strength.