Since the Affordable Care Act became law in 2010, making the math work has been a real challenge.

For Obamacare to be sustainable, the insurance "risk pool" equation has to work. Premiums from young, healthy consumers have to exceed medical care costs for older people.

Was there a problem with the formula?

"I think it was flawed," former Aetna CEO Ronald Williams told CNBC's On The Money in an interview.

"In health care you really need a balance of people who need health care today, tomorrow and in the future," he said. "And the rate structure was set in a way that those who needed health care today got the most affordable premiums. That means typically older citizens got a much better deal."

While the Affordable Care Act was being created, Williams often met with President Obama or his staff, and testified before Congress as Aetna CEO from 2006 to 2010.

Obamacare has a "structural imbalance" that resulted in higher costs for healthier, younger people, Williams said, but lower costs for those who are "older and more likely to need care."

As a result, "Younger participants in the exchanges and who purchase individual insurance paid more and they just didn't see the value, and therefore they did not come forward and sign up," he said.

Williams said that because young adults chose not to sign up for ACA, the insurance industry is "missing their premiums and that's causing the overall rate of increase to be greater."

That, in turn, is helping "make the insurance pool unsustainable financially," he said.

Since stepping down as Aetna's chief executive in 2010, Williams has been CEO of his own consultancy, RW2 Enterprises. He's also a director of American Express, Boeing, and Johnson & Johnson.

With President Trump vowing to dismantle the Affordable Care Act, the public doesn't yet know what will replace it.

Williams said what is needed is "much more competition" and lower cost, more flexible plans so that consumers have "a range of options as opposed to a 'one-size-fits-all' approach."

While we hear a lot about repealing ACA, the jury is still out on what comes next.

The process of getting Obamacare created took years of negotiating and planning from various and multiple parts of the health care system. Is the same thing going to have to take place again to remake the Affordable Care act?

"I think it does," Williams said. "We need to make changes in a thoughtful way. We need the input of hospitals, physicians, pharmaceutical companies, health insurers and consumers."

"On the Money" airs on CNBC Sundays at 5:30 a.m.ET, or check listings for air times in local markets.