My Quora Answer :
Can't really separate production of complex things into a single "waterfall" where one group do design and another do the manufacturing.
Making things is an iterative process over time.
When you manufacture you need to be looking for continuous improvements in the manufacturing process. Those improvements often require deep understanding of both the materials that feed into the process and the motivations for the final project ie. the knowledge that designers need to keep in their head the whole time.
If designers try to divorce themselves from manufacturing they end up losing sight of new opportunities in these areas. And relying more and more on the manufacturers' expertise. Until, finally, the manufacturers don't need the designers at all.
See also : MasterSlaveDialectic