People often note, like another comment here notes: > since 1960 there has been a US economic recession once every 5 to 10 years. The last one ended in 2009, 9 years ago This is an interesting line of thinking, but I think it's a mistake. We can use this fact itself and circumscribe some meta-thinking around it. Put the same fact another way, this is arguing that the 1960's started a brand new paradigm that was mater…
The lasting booms starting post WWII surprised economists of the time. Stagflation was so out-of-model that the 1970s caused a major shift in economic theory. The list goes on.
And, of course, we already know that traditionally aligned indicators have been out of sync since ~2007. Productivity and wages broke lockstep in the 70s, wage growth has lagged employment growth to an unprecedented degree since 2009, the current consumer debt bubble is overwhelmingly student loan debt which is largely non-dischargeable and impossible to repossess.
There's an entire genre of thinkpieces arguing that the economy has been doing something unprecedented since 2008, a lot of which line up with your third theory where a strong stock market is basically a reaction to weak fundamentals in other investment categories. It's weird to see that abandoned when people try to do predictions from past indicators.