The main problem, as I see it, is that people rate their bikes relative to a top-of-the-line model. In the Good Old Days, a top of the range mountain bike would set you back £3000, maybe £3500 for something very exotic. Even if you couldn't afford a £3k bike, at least you could get half way there with a £1.5k bike. Whereas now the top end bikes are stuffed full of carbon fibre Kashima-coated Ti-machined SRAM XX components that no one actually needs and they're approaching £10,000, so the bar is set higher. The half-way mark is now £5,000. People now think their common garden variety aluminium-framed, SLX-equipped bike is no longer aspirational. So we now think bikes are getting more expensive.
Bike manufacturers aren't stupid. They realise that if they build it, some people will buy it. And even if people don't buy the most expensive model in a range, they'll aspire to an XTR drivetrain and one day might splash out on at least XT bits.
We're on the edge of a precipice. Take the car market, for example. Taken at face value, the Ford Focus is all things to 99% of men. Unless you need a van, more-or-less everyone could make do with a Focus. So why on earth can you buy a car that costs fifty times as much? Imagine taking that logic and applying it to mountain bikes - What if there was some niche boutique brand that offered £50,000 mountain bikes for people with far too much money? As mountain biking becomes more prominent and more middle class, people worry less about being that all-the-gear, no-idea guy. Someone, somewhere, would buy one. And a lot of us would be lulled into thinking that our £2000 Specialized Stumpy was a cheap piece of tat.
Which, of course, is stupid. £1000 will buy you a better bike now than it ever has in the past. If we could ever stop looking at stupid 'aspirational' £10,000 bikes then we'd realise we've never have it so good.