I think there's one case where saying a plan won't work is the right thing to do, and trying to "get to yes" will often be a mistake: when you understand what needs to be done, and the (time or money) budget is too small.

If you try to "get to yes", it will often happen that the person in the position of the client will respond by cutting scope, saying "well, we don't really need that part just now".

And then it will usually turn out that they really did want what they originally said they wanted, and maybe try to sneak the features back in in disguise, and the whole thing overruns or else you end up saying no in a position where everyone is worse off than if you'd said no to start with.

I think they should teach Bridge in management school. Often not being in a contract is the right answer!