Why would you have to be limited to plots that we already know the conclusion too? You just dont have to make remakes, or at least dont make every thing bigger and more action driven. And of course, the hunger for the extremes. Of course Vulcan has to be destroyed, because nobody does art but just strives for bigger and more explosions. Thats the point. The new movies miss the point and feeling of Star Trek. They have good things, but also a lot missed opportunites and too much silly stuff.
Same for Star Wars: there is plenty of room in the established canon, and last time I checked its not a bad decision to base movies on well written and liked books. So why not make adaptions of the Thrawn Trilogy? Because Harry Potter, GoT and Lord of the Rings didnt make any money?
But... saying there's no problem with doing adaptations the books is limited to plots we already know the conclusion to. If you adapt a book, all you need to do is read that book to know the outcome. There is no secrecy, or tension, or surprises, or anything new.
If they did the thrawn trillogy we know what the outcome is. Thrawn dies, the New Republic prevails, and Borsk Fey'lya is a jerk. The only benefit for fans is seeing these things depicted on screen. Again, nothing new.
If you look up The Force Awakens now part of the excitement now is learning what the fates of the characters are. What happened to the Galactic Empire? Did Han and Leia have kids? What is Luke's fate? All the conventions from the thirty years of Star Wars lore are up in the air, and it remains to be seen what remains when it all comes down again. It's exciting to have change!
...and a relief to see some things fall off. Is anyone really going to miss the Vong that much? Waaru? Jaxxon? Chewbacca's death? 33 aby puts us past the Yuuzan vong war and into the Darth Caedus period. I will not miss those stories.
Frankly I find it exciting. It's about as exciting as re-starting Star Trek and returning to the original premise of space adventures. We're not re-treading old ground and old conventions... it's all new.