What this means is that either a) people will get a worse, cheaper form of insulin, b) everyone's premiums will go up, or c) some combination of the two. I'm guessing c).

"Insulin" as injected by diabetics is not a single substance. Instead, there are dozens of different drug products that mimic the human hormone. Depending on how well they work and the delivery method, some cost 20 times as much as others. Newer insulin drugs are both longer lasting and faster working, and easy to use needle pens cost way more than vials.

Here's a good overview of the cost differences between different drugs:

https://www.goodrx.com/healthcare-access/research/how-much-does-insulin-cost-compare-brands

Putting a cap on cost means insurance companies will decline to cover more expensive options. Those are the ones that work better and are easier to use.