IIRC the issue with insulin is that while "regular" insulin is not under patent, there are better insulin products on the market that are, and those are what everybody prefers.
Then, because most people do actually have insurance, the insurance pays for the more expensive version and so that's the only one anybody makes. The market for "regular" insulin is limited to people without insurance, which isn't a big enough market to justify all the regulatory work needed to manufacture it.
What's really needed there is to make it easier for generics manufacturers to get regulatory approval.