> Reminds me of one of my managers who said, “Sometimes, you have to let people fail.”

I often say "Sometimes, you have to let the manager fail."

Some managers don't like being told their ideas won't work. If you refuse or argue, you are seen as the reason his idea failed. I've found what works best with them is to proceed with the work, but keep them informed very frequently, so they can see how things evolve, and will be able to see the failure you had anticipated a long time ago before it is too late.

Then you're seen in a positive light, and he'll separate you from the project failure.