A blue belt of moderate size can handle 99% of the population with ease. Beyond blue belt you’re just training to defeat other skilled fighters.
Some guys can't fight period regardless of how much training they have, but most will get some benefit out of the training. You are definitely right that a decent sized trained fighter can clear out a room. I was 19 and hanging out with my friend Mike of similar age. He was nothing special physically, about 6 ft 2, 195 lbs, worked out but wasn't strong (max bench was 185 for 7 reps). He did martial arts for 5 years. Decent kickboxer, not especially fast or powerful, but he trained about 8 hours a week and that heavy bag moved. I had some weightlifting under my belt not much martial arts training.
We were walking past a schoolyard at about 11pm in Toronto around the Lawrence and Yonge area. A group of drunk teens around 17 or 18 called us faggets and such. We were outnumbered 7 to 2. The guy talking the most shit was some fat kid, probably about 6 ft 260, all around the waist. I wanted to keep walking, didn't like the odds. Mike just stopped, turned around and started walking towards them. The big group stood behind the fat kid smirking. It took us about a minute to walk around the fence and meet them. At this point I am pretty sure we are not doing the smart thing. It was 7 against 2, and I couldn't fight my way out of a lunch bag at the time.
Mike walks up to the fat kid and asks him why he is calling us faggets. The fat kid gets in Mike's face and tells him off. Mike punches him right in the middle of the face. Right on the nose. He punched the kid behind him and so on. Within a minute 6 kids were holding their faces. The last one was begging not to be hit. I learned a valuable lesson that night. Most people are absolute pussies. These kids could have circled Mike, but they froze like deer in the headlights. They barely fucking moved, except to raise their arms a little a take a step or two back. He did what he had trained to do, performing combinations that he had practiced for many hundreds of hours, over and over again. I can't even imagine what a world class fighter could do. I have no doubt that any national level mma striking guy guy could with just kicks alone, take out an average group of 50 college kids who for the most part would be deer in the headlights. Each kick sending one to the ground for minutes.
