Blue Flower

The main problem with the conventional theory

English-language-world economists call themselves Keynesian economists.

This is after John Maynard Keynes, a British economist of the early 20th century.

What did Keynes actually say?  Deficit spend during the bad times to get things going again fast(er) and then pay the debt back down again during the good times to be ready for the next time.

Sounds logical, right?

What is wrong with this picture?

It never happens!

What actually happens is

Deficit spend during the bad times to get things going again fast(er) and then continue to deficit spend during the good times because it feels so good, because the people like the nearer-term results of the deficit spending so much.

In other words, the debt just continues to build up and build up and build up and build up and build up and build up and build up.

Why did Keynes not see the error of his ways, himself, and speak up (to say, hey, we need to do something about this, I misjudged the situation)? Because he died in 1946, long before even the first implementation of his idea, along with the consequences, was fully complete and apparent. And in the meantime, there have been several attempts at implementation of his idea, all with the same result. The opposite of what he expected to see, and said should happen, in the end.
 

In terms of just the United States national debt -

The national debt doubled from $10T to $20T just under President Obama (8 years).  And it has now gone over $30T as of early 2022, just 5 years later, and is growing rapidly.