NRE into the production line, especially hard tooling for plastic is huge. Plus all those things you mentioned, especially inventory risk are often underestimated.
Don't forget that any manufacturing run itself is a huge risk. Tiny changes that need to be made to address anything from RF performance to fit and finish can invalidate a run and just burn up hundreds of thousands to millions of dollars (depending on scale)
Even for something like HDPE? I did a project a while ago that was effectively a pontoon workboat. We used HDPE pontoons in lieu of stainless steel and the cost savings was about 25%. I assume that the raw materials (basically large diameter HDPE pipe) and plastic welding tools were just that much cheaper/easier.
I was interested in doing a new HDPE product development on my own, but I'm a bit nervous this is a deep tooling rabbit hole.
Don't forget that any manufacturing run itself is a huge risk. Tiny changes that need to be made to address anything from RF performance to fit and finish can invalidate a run and just burn up hundreds of thousands to millions of dollars (depending on scale)