Many a reference to 'performance' within this debate... it's worth considering one's real meaning of performance expected from the device(s) in question. I, looking backwards with this hobby, expect nostalgia, differentiation, satisfaction and challenge, each time I open a box containing a twenty year old frame. In this regard, retro bikes deliver wholly and fully in a way a contemporary offering will never do (for me).
Do I want to ride a lap faster at MM by buying a contemporary machine - no, that has no appeal whatsoever. I do feel the need to want to pay modern prices for the reduction of 2 or 3 minutes - on the contrary, my older bikes extend the value and the challenge! BTW - I believe modern XC courses are a pale shadow of the courses we used to ride - not because of bikes and rider capability, but because of organiser liability.
Maybe if you factor outright human 'competitiveness' into your performance mix, the value of a new machine changes - I suspect most of us are not of the elite category, so what's a few minutes traded against all the other factors that make up the broader 'performance'?! Here, btw, is where we get thrills in chasing down poorer/unfit riders who have mistakenly IMHO invested in modern bikes...they would get beter value by learning/training on £150 retro bike before they blow £2000 on a modern!
Incidentally, I think that contemporary machines distance the rider from the feel and the direct sensation of the terrain. Yes, travelling faster; yes, in more 'comfort' (if you wanted comfort you wouldn't be MTBing!); and yes, accidents incrementally more serious at higher speeds. The only two apsects of modern bicycles that hold my interest are the general advancement of the breed (although there has never been a repeat of the supremely fertile period circa 1990-1992 - see particularly the MTB shows in Anaheim); and the notion that a more comfortable bike would save energy, therefore be useful for longer distances. Sadly, I don't do or have the time for all day rides, so even that performance advancement is of questionable value to me.
Lastly, think from the angle of the modern customer and the modern bicycle enterprise. Growth is everything in business - to grow you have to win new customers and also retain them - to do that, you must make customers' products obsolete as soon as you dare! Hence the mix of bogus and real advancements that annually come to market. A great example of this would be the elevated stays explosion in 1990 - virtually every manufacturer jumped that bandwagon in short order (without product testing in most cases) and now considered a poort concept, they represent excellent 'performance' to me - cheap, weirdly nostalgic, hard and thrilling to ride fast - welcome to the performance and value-led world of the retro biker!
Mr K