You seem to forget that having insurance is not the same as having health care. It is health care that people need for survival - insurance does not help with that, but decidedly gets in the way (since insurance companies make more money when people get less health care). Thus Universal Healthcare is an important concept - Universal Coverage is not.
Secondly, the folks supporting an expanded Medicare for All program are arguing for care that is free at the time of service - not free as in no one ever pays (some sort of progressive taxation scheme is clearly needed. When people have to make payments at the time of service (especially large ones relative to their income/wealth) it means that the decision to seek care becomes a financial decision rather than a medical one - a concept that most progressives find immoral.
Thirdly, you claim that no countries have the kind of system where no payments are made at the time of service - but there are a good number of countries that have exactly that.
Fourthly, you claim that the original ACA (where the Medicaid expansion was mandatory in all states) meant that everyone would be covered. That is false - as the Congressional Budget Service said at the time (5 to 6% would be uncovered). There were just plenty of exemptions that you are ignoring. Roberts’ ruling only affected about 2 or 3% of the population (of course Trump has made things much worse and knocked another 3% out of coverage).
Fifthly, when people talk about an Expanded Medicare for All program - they are not just talking about making everyone eligible for the current Medicare program - they are talking about a full service medical care program (and free at the time of service as mentioned about).
I would not dispute that there are a number of aspects of the ACA that improved the ability of Americans to receive health care and the Medicaid expansion was one of them - but it was woefully inadequate in getting at the core problems with our health care system.