Jeans are supposed to be hardwearing, 6 years life is nothing to write home about IMO.Armani jeans in the mid 90s because somebody in school usetd to wear all the designer clobber and I was trying to look hip however they lasted for around 6 years after wearing them a lot. So definitely better quality.
To answer the OP, for me it's about the expense / aura around the label, not the quality. You will get a few exceptions like brands that have build a stellar reputation for quality, but generally it's just marketing. I'm currently sat here wearing a Ralph Lauren polo shirt I bought in the US back in March 2008. I wouldn't say it's noticeably better quality than ones I've got for around £12 in Next, M&S etc.
It's kind of a chicken-and-egg situation, if some of these brands were cheaper, they'd become less desirable. Whilst they maintain high prices, I want them and look out for them in sales.