Just speaking as a blue belt, if it got to the ground in a street fight, he was untrained, and I had the top position I'm pretty confident I could finish it from there, even if he was significantly larger than me. Problem is tho my takedowns suck and we don't train them with resistance at my gym, we just drill. There's also the matter of whether he's carrying a knife on him, and also I've never experienced an adrenaline rush that comes with being in a real fight, so I dunno how that would affect me.
But... In general, jiu jitsu should work in a street fight just as well as anything else. Every standard style that you see in mma has its own set of holes when it comes to a real fight.