In 2016, Medicare cost a total of $588 billion [1], covering over 55 million people [2]. The vast majority of those people are over the age of 65 and on average, over 50% of a person's lifetime healthcare costs will be paid after the age of 65 [3]. It would cost over $3 trillion to have Medicare for all if every person alive in the United States was over 65 years old, which is obviously wrong. The real cost would be closer to $1.5 trillion if the US just paid private insurance companies to cover everybody.
> Current tax collection collects about 1.5 trillion.
No, 2016 tax collections for just the Federal government were $3.3 trillion [1]. The combined total of federal, state, and local is estimated to be at least twice that.
> If you tax millionaires at 100% you'd see maybe another trillion.
A 15-35% tax rate on millionaires yields at least $500 billion a year in income tax revenue [4]. Increasing capital gains taxes to 100% alone would net almost $2 trillion a year.
[1] https://www.cbo.gov/sites/default/files/115th-congress-2017-...
[2] https://blog.medicare.gov/2015/07/27/medicare-and-medicaid-5...
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1361028/
[4] https://www.irs.gov/uac/SOI-Tax-Stats-Individual-Income-Tax-...