I think "It's a stupid idea" is not a great criticism. It lacks detail as to why you think it won't work. I think in a healthy culture, you should be able to say why you think it won't work, even if you don't call it "a stupid idea."
But I'm sorry you are dealing with an unhealthy culture where "politeness and respect" require you to walk on egg shells. To my mind, that's not what should be happening.
I talk to my sons at times about the following idea:
I say "chair" and maybe my mind conjures up a wooden kitchen chair and the person I am speaking with envisions an overstuffed living room chair. Then we start to argue because we aren't even really talking about the same thing, but we think we are because we don't realize the word "chair" means something so different to each of us.
So I will suggest that some people here have experienced environments where communicating clearly was not "asshole" behavior and others have not experienced that. Such people can be equally convinced they are correct and failing to understand they really aren't talking about the same thing.
Growing up, I hated pineapple. But I had never had anything but canned pineapple and it was awful. While living on the West Coast, I began eating fresh pineapple, not canned. And it's incredible.
Sometimes, it's a bit like people are arguing about whether pineapple is wonderful or awful and not realizing that one party has only ever had canned pineapple and the other has only ever had fresh pineapple and neither side ever thinks to specify that detail because it seems like a given.