When someone presents an idea it's really easy to reject that idea and cite examples of why it won't work.
It's really hard to do the opposite.
One's easy, the other requires work because to be effective you have to suggest an alternative to the idea.
It makes perfect sense to simply add your opinion on why something won't work without a better one if you're standing on the sidelines - it's like watching a soccer match - it requires no effort.
But if you're the one playing soccer, it behooves you to put some effort into the debate - that is if you want to win.