One of the more surprising health policy developments in the decade since the Affordable Care Act (ACA) was enacted in 2010 has been the steady growth in the market share of the Medicare Advantage (MA) program. In 2010, around a quarter of Medicare beneficiaries were enrolled in MA; by November 2020, 43% of eligible beneficiaries were. (Eligibility for MA requires enrollment in both Parts A and B; the MA market share among all Medicare beneficiaries is somewhat lower.)
Newhouse, Joseph P. "Commentary on: The effects of coding intensity in Medicare advantage on plan benefits and finances." Health Services Research, 56.2, April 2021, 175.