Researchers did not seek to further explain Medicare Advantage's rising popularity. They cautioned that the Affordable Care Act's cuts could still disrupt the market.
"Over the next few years, it is possible there will be some shakeout in the market as payment reductions are implemented and benchmarks move closer to spending for traditional Medicare," researchers wrote.
"Ultimately, to remain viable, some plans either will have to become more efficient or modify the extra benefits they provide to their enrollees."
Private Medicare plans are run by insurance companies that contract with the Medicare agency to offer care to seniors.
In a move fiercely opposed by the insurance industry, healthcare reform cut the plans' reimbursements by about $156 billion to bring them more in line with traditional Medicare.