Apple's unified memory is great for bandwidth and efficiency and terrible for upgrades.

However, I suspect that a large portion of PCs are used as-is and are replaced rather than upgraded, much like phones or tablets.

It's worth noting that Steve Jobs was originally against RAM and other upgrades for the Macintosh. It was supposed to be an appliance like a toaster, and (most) toasters are non-upgradable. He also didn't want it to have a fan - making it more like a toaster or a MacBook Air.