Aetna doesn't add any more to the equation of CVS other than streamlining payments. It's only interesting if you use Aetna I guess, but you can generally use CVS pharmacy + minute clinic with your insurance provider today.
General high level overview of this:
https://www.bcg.com/en-us/industries/health-care-payers-prov...
https://cvshealth.com/about/facts-and-company-information
When I had Aetna, they didn't even cover getting a flu shot at the pharmacy. Maybe they changed that, but it's dumb, they covered it 100% using primary care.
To go to a pharmacy, you can stop by on your way home or do it on the weekend.
By making it inconvenient, you reduce the number of people getting a flu shot... and I would bet that more people drop out from the inconvenience, than the extra amount it costs aetna when you go to the doctors office.
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…
The truth is that everything that a payer pays for is done through contracts. Payers will often default to a specific provider because the contract covers it and they don't have a contract for other providers.
In terms of the cost, back-end rebates often bring it down. Also, this particular contract may cost more for a flu shot, but save them even more for other procedures.