Vinyl records (especially rare stuff) have been collectable for the best part of 60 years. There will ALWAYS be a demand for certain sounds be it Rock N' Roll, Ska, Soul, RnB....just look at today's groups....Kaiser Chiefs, Franz Ferdinand, White Stripes etc etc....the sound is dated back to the 1960's and so is the look. Turn radio 1 on, listen to a 'new' group and you could be back in 1966!!!!! Most of these new groups love a bit of old Soul/Motown/The Who/Small Faces and you often hear them sing their praises in interviews.
My point is this, GREAT music NEVER dies...it just goes round in circles. I was talking to a 'kid' the other day (23 year old), a fan of the new 'Mod' groups and he's getting into old vinyl (which is making a 'comeback' according to the papers....I'm sure that statement comes out every year!)
I think our collections are ALWAYS going to worth something, even in another 100 years. Just my opinion though.