I guess the ability to recognize opportunities, and stay calm enough to execute them is more important than knowing all the theory. Some people just stay focused and determined in fights while others are borderline hysteric, the same people in training can be awesome but not so much in the ring.
As for grappling in MMA, because people get so good at ground punching (special heavy bags and a lot of focus on ground striking), it changed the game, you no longer can stay on your back and hug your opponent, you need to try a submission, or a sweep and get the **** out. Otherwise you will sustain a lot of damage. I think what the OP is describing is basicaly the evolution of the sport, at first the more elaborate your ground game was the better people thought you are, later it was recognized that some basic wrestling with good defenses against submissions, and of course a great striking game while on the ground is more than enough.
Something similar happened to the striking game in MMA, at first people thought that MT is enough, later because of the low stance and the need for mobility while striking boxing came back in a big way. Today you get to see some basic boxing with basic kicks (with exceptions), i don't think it is because those fighters can't box well , i think it is the best technique for the octagon. That is why sport fighting is so effective, they keep what works and get rid of the fancy ****.
I think it comes down to physical prowess and toughness.
When you take two untrained people and let them go at it, the most physically imposing, toughest guy will usually win.
Theoretically, you can have the same situation with two expertly trained people. Assuming the same high level skillset, I'll put my money on the most athletic and toughest fighter.
Of course that's simplifying the hell out of things because skillsets are rarely equal. And then there's luck. Even a blind squirrel finds a nut sometimes. It's been demonstrated time and time again that when you put two grown men in a cage with little gloves anything can happen. It only takes one good punch to give you a completely unexpected outcome.
^This book has an interesting outlook on devising and measuring improvement in martial arts, based around Six Sigma/Kaizen principles.