My guess? Not that many. In my house I have two desktops and one laptop PC, one tablet, three smartphones and one dumbphone. The phones and tablets are all newer than the PCs, but we still use the PCs all the time. I just don't need to replace/rebuild my PC every 18 months like I did most of the 90's. Even gaming is fine on a computer several years old if you are willing to play on less than MAX everything. Games do a much better job of scaling. The new games are going to look and play pretty much as well as they did when your computer was new, they just are not going to look any better. Oh, and there are probably at least a couple dozen great games on Steam for $9.99 that you didn't play 4 years ago that will go great with you 5 year old machine.
The PC isn't dead, it's just a mature market.
This is pretty much it. I remember back in the lat 90s/early 00s, when I upgraded the motherboard/CPU/graphics card of my gaming machine about every half year, because the newer hardware actually gave a huge benefit in gaming performance. I remember not being able to run new games at decent settings until I got the next upgrade, because you constantly kept running into hardware limitations (CPU not fast enough, graphics card not good enough to run at more than 800x600, etc.).
And there was tons of new hardware which actually gave benefits (from the "this new screen can run 1024x768 at non-interlaced" to graphics cards switching from ISA to VLB to PCI to PCIE, to graphics cards actually using 3D (old 3dfx cards, then Nvidia TNT and so on) to RAM increasing more and more to hard disks increasing in size and so on).
These days, it does not really matter much if you have 8GB or 16GB RAM, if you have 1TB or 4 TB disk space, if you have a Core2 Duo CPU or a current i7, if you have a GeForce 680 or still a 460. It all still works fine, more or less. Only if you want to play the absolute latest 3D action shooter in the highest resolution at the highest details settings - then you actually need a high end CPU and a high end graphics card. But there are not that many of those games around, and most of the time they are not actually fun to play anyway (except for the short "ooooh nice graphics" moment). So why upgrade? There's no need to anymore, except if you actually like building PCs or if your old hardware is acting up and you need to replace it because it is failing.