In this "sport", we already have one guy pinning down someone for his buddy to soccer kick and stomp on his head. How much more realism do you want?
And are you saying that there are no rules in the streets, even unwritten ones? I've been in over 10 street fights, and of the ones that I won....I never went for that extra goal of manslaughter and/or second degree murder....hell, not even malicious maiming. Have you? I have at least 1,000 videos of real street fights, categorized. And only about 0.010 percent, involves death....and half of these deaths were accidental....almost all due to a 1 punch KO where the guy cracked his head on the cement on the way down, and died at the hospital.
People rarely to almost never, fight to the death or even maiming in the streets. They stop on their own or the fight gets stopped by bystanders. Many times, it's the Winner who stops on their own....usually when someone gets KO'ed. Also common is when the losing guy says he's done and the winner cease the beatdown.
You think that trained MMA fighters can't cheat or do whatever they wanted in a street fight neither? Now you have someone much more: stronger, younger, resilient, precise, trained, experienced, etc. that can also cheat with weapons, improvised weapons, etc. vs. a regular old dude. Sure, the old dude has a chance just as long as he doesn't think that he's the only one with a gun or something.