If it was developed in 1996, the patent protection would have expired by now. If the cost of production was "steady", then the question is why is a competitor not jumping in to make and sell something that would earn them a huge profit margin?
Otherwise, you seem to be repeating the old economics joke:
"The young economist looks down and sees a $20 bill on the street and says, “Hey, look a twenty-dollar bill!”
Without even looking, his older and wiser colleague replies, “Nonsense. If there had been a twenty-dollar lying on the street, someone would have already picked it up by now.”"The world has many very smart people spending many hours working very hard to find these opportunities, so the likelihood of it just sitting there like a $20 bill on the street, and not being picked up, seems quite small.
I do not know the answer myself, so I am interested in finding out. reddicky posted this:
https://www.hopkinsmedicine.org/news/media/releases/why_peop...
which sort of answers it:
>Biotech insulin is now the standard in the U.S., the authors say. Patents on the first synthetic insulin expired in 2014, but these newer forms are harder to copy, so the unpatented versions will go through a lengthy Food and Drug Administration approval process and cost more to make. When these insulins come on the market, they may cost just 20 to 40 percent less than the patented versions, Riggs and Greene write.
So it looks like there is some expensive expertise involved and investment into R&D that needs to be done.
In theory there is 'nothing' stopping another company from coming along, even a group of Biohackers may try to do something - I wonder if that's been done... :)
I would imagine that even other companies already in the pharma space would be interested. They don't want to play the race to the bottom game with billions of dollars in upfront costs. If anything, it's probably more profitable to make a new form of insulin that you get to patent. Which is how we end up where we are.
The insulin in question is Lyumjev from Eli Lilly.