I'm not sure why many here assume that an MMA fighter would instantly pull guard in a street fight

I wouldn't pull guard in an MMA bout, let alone in some bar or parking lot. Most people have no idea how to fight and simply punching them in the face once ends the scuffle. If that doesn't work, a trained MMA fighter has the option to punch, elbow, knee & kick. And of course, you can always take someone down and G&P the shit out of them. I would bet 99% of people who find themselves on their back in a fight, would panic and end up giving their back.
Of course, in a street fight, soccer kicks, knee to the head, head-butts, 12-6 elbows, etc are all available. I firmly believe that a trained MMA fighter would do better in a street fight than a trained boxer.
Speaking of boxers, this is a great video: