Let's separate the reality from the political spin. A number of factors are contributing to the related phenomena of premium rate increases and carrier market exit. First, the marketplace risk pool is smaller, older and sicker than many analysts anticipated – not always for bad reasons. For example, fewer employers are dropping coverage than CBO expected, making the universe of people eligible for marketplace plans smaller. Other factors driving utilization in the marketplace plans include pent-up demand from the previously uninsured, the extension of certain non ACA-compliant plans that has kept what is likely to be a healthier population out of the ACA marketplace, and the gradual phase-in of the individual mandate, which has probably led some people to decide that going uninsured is still a better deal economically. Congressional efforts to undermine the ACA's risk stabilization programs have not helped either. And two of those programs are expiring in the next year, leading to a one-time premium bump.
In the case of Aetna, there are carrier-specific factors at work, as well. In particular, there seems to be a link between the prospects for Aetna's merger with Humana and its decision to pull out of the marketplaces. In a July letter to the Department of Justice (DOJ), Aetna warned if the merger was blocked, it would pull back on its marketplace participation. The announcement of the pullback came shortly after DOJ sued to block the merger. If you are a complete cynic, you would see this move as either retribution or, at best, a negotiating ploy. It's probably closer to the truth to say the company expected to make a lot of money from the merger and needed some regulatory goodwill to pull it off. In the absence of that goodwill, and with the merger now in doubt, they became less willing to sustain losses in the ACA market.
An understanding of the underlying dynamics leads to the conclusion that some of the factors that are causing the current turbulence will simply abate over time, even if no action is taken.