Good value/cost in comparison to other developed nation's public healthcare solutions. Isnt that what the OP is comparing healthcare in the U.S. to?

Medicare is also very expensive, it spends less in administration but also has very high costs in regulation. Medicare has the size of public healthcare in other developed nations, and its still high cost in GDP/capita. So expanding medicare would not solve the cost issue, it will just swipe it under the rug.

Medicare spends less than private insurers; despite the bureaucracy and regulations it's far more efficient than private insurers. Something like 98-99% of every dollar spent on Medicare is spent on treatment vs 85% for private insurers.

One of the reasons that many doctors hate Medicare is the paperwork associated with patients. (This paperwork only exists because of the bad actors who bill Medicare for non-existent health services.) But they stick with it because unlike private insurers, Medicare will pay out for services legitimately performed, which is why nearly every doctor who drops Medicare patients ends up taking them back within 2 or 3 years.

Its still very high per patient, higher than developed countries. And medicare will not necessarily get more efficient as it grows, in fact, the opposite.

Should we remember what the healthcare.gov for obamacare turned out? Billions into a non functional website.

Its true medicare shows some good signs in comparison to private, but it shows some other terrible ones. Lets not forget that healthcare regulation is designed by the same people that run medicare, in a broad sense.