The PC is not dying
Sales are down simply because buying a new PC is no longer something you need to do every couple of years just in order to run the latest software (let's ignore gaming as serious PC gaming tends to be a niche).
My desktop PC at its core is 8 years old. All I've upgraded since the original build is storage (SSD + larger HDDs) and the graphics card (the original was cheap and basic). But at its core, everything I need to run still runs and generally very well. The only exception being audio and video rendering, where it is starting to show its age.
My laptop is two years old and as fast as the day I bought it. It should last me several years
I still use both over and above any other tech. Tablets are no good to me - I like to (properly) multitask. My phone gets slower by design (forced OS upgrades) and thus I MUST update it every couple of years (just like the olden days of desktops) else face more and more frustration.
I don't believe the PC is anywhere close to a death spiral even in the home environment. Consumers just don't need to be spending their money on it as often, so the stats are skewed.
Developments like the Surface are just designed to find a new niche or reason to part people with cash. Just like the iPad finally got the tablet right, MS are looking for the next revolution/evolution.