I THINK HEINLEIN KNEW THE ANSWER: A scarcity of economic growth.  From the article: “The causes of the productivity collapse are unclear.”
… if you don’t read Heinlein.  If you do, you’ll remember this:

Throughout history, poverty is the normal condition of man. Advances which permit this norm to be exceeded — here and there, now and then — are the work of an extremely small minority, frequently despised, often condemned, and almost always opposed by all right-thinking people. Whenever this tiny minority is kept from creating, or (as sometimes happens) is driven out of a society, the people then slip back into abject poverty.

This is known as “bad luck.”