It's not a surprise as a PC usability-span gets longer the rate of replacement gets lower.
Could it be that casual users can do all their tasks on their phones/tablets now compared to 10 years ago, while the enthusiasts build their PC:s from scratch, thus hollowing out the OEM PC market, leaving only laptops?
I too think that's exactly the point. Average user doesn't need desktop PC anymore and buys smartphones, tablets, consoles instead, while power users and gamers usually build their rig from srcatch. Also, there's no need to upgrade PC every two years as technologies (maybe except GPUs) doesn't evolve as fast as it was used to. TL;DR it's not PC market that's shrinking, but a branded PC market.
Further, now even smartphones are more powerful than average users need, so e.g. the iPhone 5s is still selling well.
Even the laptops got used until they either died or in one case, got stolen.
We are way beyond the need of changing computers every three years.
I'm switching back to a desktop because I rarely use the portability of a laptop.
It's a very good system for running state of the art deep learning experiments on now. Cost me ~600 for the upgrades last year. You couldn't even buy a laptop with these specs. I bought a steam link and 2 controllers for ~150 last year to play co-op games with my wife, much cheaper than a console with the added benefit of my existing steam collection and Steam holiday sales!
I may have gotten lucky, because I sure as shit don't know what I'm doing picking components, but the fact that I can get lucky probably means there's a way to pick things with headroom intentionally.
For example, recently I got a 2011 PC from someone that didn't want to keep it on its new flat, and I cannot upgrade the GPU to a DX12/Vulkan one, because they all require PCI Express 3.0 and the motherboard only supports v1.0.
It is cheaper to get a whole new computer, like a NUC, than starting the cascade of updates.