This is me letting you know that Keynesian economics solved deflation, making it less difficult to get out of market contractions (Note, the comment you refer to specified specifically deflation to be an issue, and did not claim that it solved bubbles and poppings completely nor reduced their frequency). The last time the US had faced significant deflation has been 100 years ago. We used to face deflation much more frequently before Keynesianism since our country’s founding.

Inflation sucks but deflation is a death spiral. There is some more nuance, but most economists would agree that deflation is a lot scarier in general than inflation due to its feedback loop.

Moreover, the reason why we keep getting rampant inflation is more due to neoliberalism and neoconservative economic/fiscal policy rather than modern monetary theory and Keynesian monetary policy. If anything, these monetary policies have massively reduced the amount of inflation we have been feeling for the past several years.