I've trained grappling (jiu jitsu and judo) in real life, so I wanted to add my experiences here for perspective.
Disclaimer: sometimes the real world way a thing works isn't the most fun implementation in a game, so I'm not necessarily suggesting that.
Grappling in real life is very, very, very skill dependent. If you have no training, and grapple with someone who has even a few months of training, it is realistic to say you cannot possibly win, and that you will be in extreme danger almost instantly.
I've witnessed skinny, trained 13-year-olds defend themselves easily, indefinitely, against 25-year-old weightlifters with no training, and who were visibly angry that a 13-year-old was making them look foolish. I'm not a large person, and I've easily controlled people much larger and stronger than myself. In my experience, an untrained person needs to be about 30 kg heavier and be very fit, in order to have a small chance to get out of my holds with brute strength. In many cases, if I really wanted to hurt them, they still wouldn't be able to muscle out without being injured (granted, this is without them being allowed to try striking me, which might help or hurt them).
It's realistic to say that most untrained people will commit a fatal mistake in the first second of grappling. A slight repositioning of an arm, a slight shift of their weight, and they've put themselves in an unrecoverable position, completely unaware until a moment later when they can't move anymore. Once a skilled grappler has you in a bad position, you cannot get out. They can now do whatever they want to you, with very little effort. The physics of the arrangement makes it so you can't use your limbs, or you can't breathe. It's like trying to pull open a push door; it just doesn't work.
It's similar to an untrained chess player going against a master: it's likely that in the first few moves, the untrained player loses the game and doesn't even know why.
My experience is that it takes about a year of grappling training to be able to defend yourself briefly against an experienced grappler (until they eventually get past your defense). It takes a few months of training to be able to easily immobilize and control untrained people.
The disparity in skill levels is always enormous. When I used to train against a black belt, I felt like I couldn't put up the slightest resistance. Nothing I tried worked. It was like being untrained. Then there are world-class black belts who can easily defeat local gym-level black belts.
TLDR: grappling in real life is a skill that takes years to master. I'm not saying grapple should be a skill in this game, but my experience is that untrained people are immediately and completely at the mercy of trained people.