For me trying to negotiate with people playing super hardball (ignoring social norms etc) makes me think they will not honour any agreements made. Because those rely on social norms too. Curse your sudden but inevitable betrayal etc.
I have met tough negotiators who uphold their end of the deal just fine.
The people who let me down the most are those who do not even attempt to negotiate on my prices: usually they'll think about it way later, but still won't accept discounts, until they run out of budget. Then you're thinking to yourself “why did they undermine this whole thing by paying me more than they could afford, for something I was willing to negotiate on?”. Then they miss 20k in billing, and you break it off.
At the end of the day, it's a piece of paper. Enforcing it in a civil court will take years, if not decades and costs tens of thousands if not hundreds of thousands of dollars in legal fees. Most people don't have the time or the money to do this, so it ends up "settling", often times with arbitrary fairness.
Settlement is more a game of "who can do what to whom".