I think your analysis of the PC game market is off the mark. Saying that a low-spec game still selling well means that high-spec games are of poor quality is flawed.
The Sims sells much better than FPS games such as Half Life 2 and FEAR because The Sims has a much broader appeal, that doesn't make Half Life 2 a sucky game (I cant speak for FEAR as I don't own it). There's still good reason to get a PC capable of playing future games such as Oblivion, NWN2 or the next Half Life.
In terms of the scale of sales, there is a definite decline that I have noticed in high street stores stocking PC games, gamestation near where I live doesn't stock any PC games outside of the chart and preowned sections, this obviously must be due to the companies seeing PC games as a weaker market though I can't claim I know why.