Oh boy, this discussion could take a while....
Look, man. When you get into a fight, it means you and your opponent are gonna throw your limbs at each other, also called punching, kicking, elbow'ing, knee'ing or headbutting. If this is not happening, then it means you are gonna grapple (either standing or on the ground).
That's it:
1. throw limbs at each other
2. grapple
MMA focuses on both of these. I'm not saying KM is useless but it tries to find a shortcut around the above basic elements of fighting. Don't just throw leg kicks, but kick right under the knee so it will break... Don't just punch, but punch your opponent's adam's apple... Don't just grab him, but bend his arm so the elbow breaks. Etc. etc.
Newsflash: You're not gonna pull those things off with a moving person who is actively engaging you in a fight!! Go ahead, try kicking his balls. Oops, your kick was one inch to the right of his balls and it just hit the inside of his leg. Which hardly has an incapacitating effect, so right after your kick landed, the opponent threw a punch directed at your face. It lands... You go down... You wake up in the hospital.
"But hey, I learned deadly moves at KM training? Why didn't I win?"
Seriously, not even a Shaolin monk training from when he was an infant has the precision to hit a small specific spot on the opponent, like eyes, balls, adam's apple, knees. Perhaps if the opponent stands completely still or moves as slow as a turtle, you'll succeed... But a moving person, no way. And sure not with FORCE. Take a marker and draw a spot on the wall, then try hitting it with force, put some real power in your punch. Not as easy as it seems.... You'll see people first hitting that spot in slow mo a few times to get a feel for where the spot is and how to throw the punch, then eventually doing it with force.
Guess what? You can't do that in a fight, lol. And we're talking about a stationary spot here, not a moving person!
Long story short: It's very unlikely that you hit those spots, and even if you did somewhere in the fight, it won't be the first punch or kick that you throw. So there will definitely be an EXCHANGE of punches, kicks etc. And that's what kickboxing (basically the standup part of MMA) focuses on, and it does that better than KM.
When you and your opponent get closer to each other during the exchange of pucnhes/kicks you'll end up grabbing each other. Ever watched a boxing match and counted the times they get into a clinch? Boxers do very little damage from the clinch, they want some distance so they can throw their punches at each other. And STILL they end up in the clinch very often! That's just what happens when you're in a fight... So then you either take your opponent down or the other way around. Which MMA focuses on, much more than KM!!
I could go on for a while but whatever. You'd be a fool to choose KM over MMA if you want to improve your chances of surviving a streetfight...