Keynesianism is / was simply an effective way of stimulating a moribund economy. MMT is nothing more than tons of free helicopter money. And re Marxism vs Communism, you are so correct.

And let's also note that socialism and communism (particularly the Soviet style) are entirely different systems. Nowadays, clueless idiots often conflate the two, usually in order to fearmonger. But while there are no examples of successful communist states, there are plenty of examples of well-run, successful socialist states.