There's a video I watched a while back by Jesse Enkamp (the "Karate Nerd" as he calls himself):
I really like the way he puts it. Now, the analogy starts to break apart a little bit if you compare all of the different styles of fighting. But let's take a boxer, a Taekwondoist, a BJJ fighter, a wrestler, and an MMA fighter.
As beginners, they're all learning the basics of their style. The boxer is doing 4 punches over and over and over. The Taekwondoist is starting with forms, but primarily will focus on 4 basic kicks over and over. I don't know what a BJJ fighter would be learning as a white belt, and it's been too long since I did wrestling to remember what the first few classes cover, but I'm sure they would be learning the basic foundations of their art as well. I'll guess as well for the MMA guy, that he's going to be learning some basic strikes and/or throws, depending on which class he shows up to.
As intermediate fighters, whatever it is they are stereotypical for, they will at least look like they know what they are doing. Going down the list in order, they'll have decent punches and combinations, decent kicks and footwork, a few decent locks and chokes, a few decent throws and pins, or a little bit of everything. They're starting to put the pieces together of what they've learned.
As advanced fighters, they are capable within the scope of their own competitions. The boxer will know how to throw combinations and defend against punches. The Taekwondoist how to read kicks and set up the headshot. The BJJ guy will know how to outmaneuver his opponent on the ground, and the wrestler will know how to control his opponent. The MMA guy will have figured out his strengths and weaknesses, and know where he wants the fight to take place, and have some strategies to keep it there.
Now it is at this point that most people compare the arts. "BJJ is the best because all fights go to the ground." "MMA is the best because it can deal with every situation." "Taekwondo has the best kicks, but that's all they know how to do." Well, at this stage, you are advanced in your areas of focus, but at best intermediate at the other areas you might find yourself in.
As expert fighters, now they are so competent in what they do, that they can make sure the fight goes that way. The boxer and the Taekwondoist can use their footwork and timing to keep a grappler from even getting close. The wrestler or BJJ will know how to slip through those strikes in order to take the other person down. And the MMA guy has simply fought enough people that he'll know what to do in almost any situation.
So where The Karate Nerd has a mountain that everyone is climbing, where everyone will learn all of the techniques, I don't think that's quite the case. However, whatever your chosen fighting style is, the better at it you are, the more you can control the fight. If you're a boxer, and you know how to close in on a kicker, or you can react quick enough to a grappler, you're going to win. If you're a kicker, and you know how to dance, you're going to keep people at range. If you're a grappler, and you know how to close on a striker, it doesn't matter if you know how to strike or not.
But it takes pushing beyond the phase of simply being good enough in your arena, to being good enough at what you do to survive in any arena.