Historically, large expansions of health insurance for some tends to reduce access to care for others. Just after universal health insurance was introduced in England and Wales in 1948, receipt of care increased for most of the population, but it decreased for people with high incomes, precisely those who probably had good access before universal coverage. A similar thing happened in Quebec after Canada introduced universal coverage. Physician visits increased in general, but decreased for higher-income residents. Waiting times also increased, and more so for higher-income groups.
I am unaware of any similar research pertaining to Medicare. When it was introduced in the United States in 1965, access to care for those 65 and older improved, but we don’t know whether access to care suffered for the rest of the population. Medicare payments at that time were generous, and that may have helped spur the expansion of supply to meet the new demand.
The same cannot be said of the Affordable Care Act, in general. Indeed, it is financed in part by cuts to Medicare. However, the law does include an increase in funding for primary care training and in fees paid for primary care visits under Medicaid, albeit only through this year (Congress is considering extensions).