It depends on the situation. Many MMA matches have been decided on one technique, be it a punch, kick or throw.
Sure. When you're preparing for a competition, you spend hours identifying your opponent's weaknesses and deciding how to exploit them. Same with judo. Same with chess, for that matter.
Which just shows how different competition is from any kind of real situation. Planning for a known opponent is very different from dealing with a random attacker who happens to train in a given art.
And if you spend three rounds trying to get an opening for that one deciding technique, can you really be said to have "neutralized" your attacker?
(I know, I know, Ronda Rousey armbar specials. But even those don't work on everyone.)