If consumers are paying $300 a month they might decide to take a cheaper insulin instead, move to a generic if possible, maybe even get their type 2 diabetes under control before they need insulin, etc. Now that it's capped for them - why should they care? Their insurance will pay for it.
Because how much insurance companies can charge is tied to how much they pay out, they now get to raise their prices and profits. Because they all need to compete with each other, they normally couldn't do this all on their own. This is essentially government mandated price fixing - or perhaps another term is better?
There's absolutely nothing stopping the insulin manufacturers from raising their prices even more now. They'll want to slowly boil that frog just so they don't come under pressure, but that's fine by them.
Finally, this is good for the Democrats because most people don't realize the above and think they price was actually capped to $35. The comments on Reddit about this show it pretty well, calling out Republicans for not voting in favor of it. How dare they? In the end it'll be great for them in two ways, because insurance prices are going to keep going up to ridiculous levels until people decide that we need to move to a socialized system.
Medical prices are out of control in our country and the lack of price discovery is exactly why. I've commented before on this, but when I was prescribed testosterone I was put on Androgel. Androgel was $400 a month. Injections are something like $10 a month and work better in every way. There's now a pill you can take for the low cost of $1,000 a month. If people actually had to pay that cost up front, they might decide to go through the hassle of injecting themselves once a week or two.
We either need to making insurance in to actual insurance again, meaning a very high deductible, or we need price controls. Anything else is a terrible idea. Mandating high deductibles is probably a no-go politically, but price controls would be easy. Cap the costs of medications and procedures/visits to the average of 5 other first world countries. Bam. Done.