stuff gets bought after advertisers/magazine reviewers have suggested that it's better
Hmm, well we're onto the psychology of advertising, which is somewhat complex I think. For example, why would they make Marmite adverts that tell you it might make you puke in a policeman's hat? What they do is subliminally plant an association in your brain between their product and something positive. That might be a desirable image of shredding trails, or a sexy woman (it's always women), or something funny or unusual (hence the Marmite ad).
So the bike ads aren't actually telling you the new is better or faster than the old UNLESS it's an actual technological change like say the Gary Fisher G2 geometry ads; rather, they're making you want something but you don't know why. Lots of people on here say stuff like 'I don't know why but I really want this'. It's not necessarily specific advertising, but a culture that's been fostered.
I agree advertising works, and we are all manipulated to an extent by it (even the Rivindell bloke) in all walks of life. But I don't think it's fair to claim that most people really think that the latest bit of kit will make them faster or a better rider. After all, I'd say most people on here don't even know their personal best time for their local trail centre So how would you know if you were faster?