Is your experience really that engineers cannot reliably estimate feasibility given a fixed budget, and that they have to be treated like children and asked for something completely different?
If they can't do their own feasibility assessments, they will be producing bad work, because there are many feasibility questions that arise during design and development, and never even make it to the parents^Wmanagement.
Then I feel like that's the bigger problem that needs to be handled with training, and perpetually working around it is still a bad idea.
