I don't think that's it. Gaming PC's generally cost more than a console, and the "general purpose PC that can also dabble in some gaming" is becoming less common. It seems that people are buying less PC's but those who are still buying them are often buying them for a purpose.
I think it's that "gaming" (and by that I need AAA high dollar value gaming as opposed to casual cell phone/mobile device gaming) is becoming a little more niche of a hobby. Niche hobbies often have high costs associated with them because the small group of people who are willing to participate are willing to unload large sums of money into it.
PC gaming has always been where the best performance and visuals have been available - and it could just be that the remaining customer base are the ones who want that whilst more casual people are fine using their mobile devices for playing a different type of game.
I will say personally I've always bought consoles strictly for exclusives, while always also maintaining a gaming PC as well. As exclusives become less of a thing and everything seems to be available on PC anyways, I have little incentive to actually buy a console anymore.