Marketing, producing products your users want do matter. Part of that is convincing your users they want to do that... There's a much more fundamental thing going on here. It's the same force that killed the dominance of the Mainframe, and the Mini.
Machines are fast enough now to do everything most consumers want to do with them.
Let's extrapolate what that really means:
1) Surf the web - Fine on 5 year old hardware (Even with stuff like Flash)
2) Watch video - Fine on a 5 year old box
3) Write something - Fine on a 5 year old box
4) Compute something - For most users fine on a 5 year old box.
Do you see the pattern here? The average user can do everything they want to do. Which gets to the crux of it. The real problem is that their isn't some new killer application that causes such a stir most users want to upgrade. The same thing happened to the mainframe and the mini. Once mini's were fast enough people used them to do many things (not all things) mainframes did. When the killer micros were fast enough users used them to do many things that mini's did.
What's going on is that we lived through a period where there were tangible benefits in upgrading. The truth as much as some might argue about it is that users bought into needing multitasking, they could see that editing some document or image (or video) was tangibly better on a faster machine.
What did that? Think about the mid 80's to 90's and even early 00's. There was the GUI. 90's we had the Web. Do you remember how insanely saturated EVERYTHING was with the web? It was www this and www that and web this and web that. In the 00's there was video. Hey you can watch video on your machine over broadband.
The machine running Windows 95 really was better for surfing the web than their WFWG 3.11 machine. The windows XP really was more reliable than the Windows 98 machine if you wanted to install random crap.
What's happened is that there's no real new apps that the average consumer cares about.
Now, if someone comes out with a killer app that needs a machine of PC levels of capability and piles of consumers are convinced they need it then the PC will take off. Microsoft has made Windows 8 look like a tablet OS to the average consumer, who based on sales figures is not convinced at all that they really want a tablet OS on their PC.
The PC won't die, just like the Mainframe didn't die and yes we still run piles of Mini's (Servers). It's just that your PC may be the size of a raspberry pi. For some people it will be the size of a regular PC or Laptop. Guess what - if you have to stick a decent sized screen on it and a keyboard it's going to be at least some size. That's until you either wear VR style glasses (No need for a big screen) and don't have motion estimation stuff so good and cheap you can just waggle your fingers in the air like you are typing on a virtual keyboard.