But if just one of the people insured by Aetna ended up in the hospital due to complications from the flu due to not getting a flu shot, the amount of money they'd have to pay for the hospital stay would pay for hundreds of flu shots.

The insurance provider doesn't care how much it costs. The ACA states that the insurance provider must spend 80% on services. So as far as they're concerned, the higher the costs of healthcare the more money they get to keep. They just can't let costs skyrocket faster than they're able to increase your premiums.

The market is a horrible contrivance, and insurance is fundamentally unworkable in a market, unlike actual services and products where you know what you're getting and when you'll need it…

But that said, do you have even the tiniest bit of evidence that insurers actually do what you're saying in order to keep their costs up per ACA requirements??

There's all sorts of shenanigans in the industry, but your speculation sounds far-fetched. Remember Hanlon's Razor…

Occam's razor is a better fit here. In a world where they have to return any cost savings to the insured, what is the incentive to reduce costs?

In theory, competitive rate setting for premiums. Hasn't worked out that way in many places though.

This is the fundamental flaw in the ACA. It did nothing to increase competition. In fact, increased regulation pushed some competitors out of the market. I can't tell whether this was a fool's errand or an intentional destruction of the market, pushing us towards socialization.