It could be that there's no form of submission in the rules. Technically, there are techniques when grappling, like in Jiu Jitsu, where not only is the person completely unable to move, but they are in so much pain that they will do anything to get out of it. And technically, if you decided to ignore their cries, then you can break bones rather easily, once you get them in the right position. With the right leverage, it may even be possible to rip limbs off.
None of that is in the rules. The rules just say that if they're grappled, their speed is 0. I can't say "I put him in a choke hold for two rounds". A proper choke hold will put a person unconscious in about 14 seconds. That's just over two rounds. They basically get three Athletics/Acrobatics checks to break it. The first one when the choke hold begins, the second and third at the end of each of the next rounds, and then they're passed out. Keep holding on and they'd fail their death saves automatically. Or make a wrenching motion, which a grappler would know how to do, and break their necks. People are surprisingly fragile if you know how to twist correctly.