"The new plan is stupid" is rude because it's not actionable. It doesn't say what specifically is wrong, why it's wrong, or how it could be better. The older I get the less tolerant I am of people simply venting or breathing out generalised criticism that doesn't help us navigate to a better place.

I expect people to bring criticism, but I expect them to do so constructively if they want to be listened to. "This plan is unworkable because X but if we did Y instead that would lead to Z which would work much better," is actually helpful. "This plan is stupid," isn't.