Impressive article. He gathered a lot of data interesting in its own right, tested a lot of theories, focused on facts over assertions; and it did it in a way that was a pleasure to read.

The conclusion was somewhat underwhelming: it's a least two things hitting at once: inflation and COVID, possibly with social media thrown in.

I dunno if he's right, but I'd probably add two more factors: the latest round of the ongoing (for 4 years now) Ukraine war coincided with the start of the decline, and now the rise of AI providing a sting in the tail. In fact it was the total lack of AI writing in this piece that made it such a pleasure to read. It's a rare find nowadays.