It's true that you can upgrade PCs, but I've also seen that it's really not needed in consoles, since games would naturally scale down for them during their lifespan. Your console probably last about as long as a proper computer.
The PS3 retained 2006/2007 and is being replaced later on this year. My i5-760/HD5870 rig has been with me since around 2009/2010 and I think it's about due for a complete upgrade (as far as cpu/mobo/gfx is concerned) sometime early 2015. The cost of the PS4 is around 500AUD here, and I can't imagine the parts upgrade costing anything less.
The besides the cost here, there's also the cost of running a computer for general use. My monitor/PC idles at around 110watts, and with my sound system plus light cpu load it usually draws 110-150W of electricity. Using the same sound system on my laptop would draw.. 40W max? Electricity's costing higher and higher these days, so it's definitely something to consider.
Gaming PCs have their places and reasons for existence, and cost is one that seems to be diminishing. Granted, some games are simply horrible to play without a mouse/keyboard..