I'm not quite sure that it's a reward for misbehavior, though I'm sure that at least in the short term it will feel like one. If someone pulls all that crap and gets what they want right now, but everyone else decides they're just done and walks away.... It may not be a problem initially. But at some point, they're going to want something, or need help, and no one is going to be there.
Also, I don't care if their bad behavior is rewarded. I can only control and influence my life and if the decision makes my life better and easier, I don't care what it does to their life. If choice A is better for me than choice B, I will choose A and if that means the other person ends up with $10m dollars? Thats irrelevant because I could only choose between A or B for myself. I'm not going to make my life worse by going with B, just because I want to make sure their life isn't made better by my choice, regardless of what I think they may "deserve".
(Clearly, in healthy relationships with reasonable people, choosing between A and B isn't always about just what is best for only one's self. But when you no longer care how your decision affects them then all you can do is look at what is best for you. If that happens to make their life better too, it is irrelevant.)
And to Sibley's specific point, I also agree. When I've considered the possibility walking away from an inheritance, it has always been with the idea that if it comes to that it will be, "Congrats, everything is yours. You are also now the executor and even though you are bumbling idiot, you have to sort out, on your own, how to manage that and what is legally required including paperwork, tax filing, and everything else. And, lose my number. This is the end of the relationship. The $100k [or whatever the amount is] you are receiving is the cost of our relationship which is now over in all ways." And for someone like this person specifically, that may not feel meaningful initially, but they are not good at life and have spend their entire life being dependent on other people. Even something like filing their own, very basic taxes has probably always been handled by a family member. (And yes, this is an able-bodied and able-minded adult. They just dig in and remain intentionally ignorant on so many aspects of basic adulting.) So it likely will be problematic for them. When the money is spent, which will happen quickly, they will want a hand out. When they can't figure out how to tell various entities that the person has passed away, in order to get access to accounts or proceed with legalities, they will want help. When they need to sell a house but don't know how, they will be at the mercy of whatever real estate agent they find. When they decided to invest everything in Bitcoin, not even understanding the most basics of what Bitcoin is, and they lose it all? Not my problem. In this scenario, I would not take those phone calls because this person would be basically a stranger to me. Less than a stranger, in fact, because a stranger is neutral and hasn't squandered any good will or decency.
So sure, their bad behavior might be rewarded with more inheritance, but that will hardly be the boon they make think it is.