The actual Medicare for All bill in Congress as we speak has plenty of provisions to increase supply. Minimum nurse-to-patient ratios, minimum physician staffing levels, maximum wait time standards, "health professional education expenditures sufficient to meet the need for covered health care services", a capital expenditures budget to improve facilities, etc. I don't think we need to debate the details here though, we all agree that more doctors and more hospitals would be necessary.
"Medicare for All" is a convenient shorthand, it doesn't literally mean "just let everyone sign up for Medicare and change nothing else".
> Market demand for healthcare services is essentially infinite.
This is kind of an insane claim to me, can you back that up somehow?
