It's important to remember
PC use isn't declining, only PC sales. At the moment the roomie's nephew is staying for 8 weeks. He was commenting on the really nice case I had. I said it was the last hand build I did and when I built it, I finally said to hell with budgets, I'm building what I want. So I got myself a nice case, top end power supply, maxed the RAM and got a quad processor. It started with a high end graphics card as well, but since I don't play MMO shooters, I didn't see the need to replace it with equivalent when the old one died. Then I had to look up when Vista was released to date how long ago I built the PC. Vista was just being released at the time and I built myself a dual boot XP/Vista system which was migrated to XP/Win 7. That was about 8 years ago.
I might build myself another new system after Windows 10 is released (mostly so I have a system that will last about 8 years for Windows 7), but that will depend on how much I spend when I finally replace my now 14 year old car.