Apologies for the ramblings....
Bike prices in general have gone mental, look at Honda C90 C50, you had trouble giving the bloody things away a few years back, now a worn out piece of crap can fetch £600, depends on type too, you can probably get a spotless, low mileage GSXR less than a worn out, old BMW that needs money thrown at it.
Nobody wants the plastic rockets anymore, bikers in general are getting older and wanting to return to their old rides, they can now afford 2 or more bikes, something else that can be driving up second hand prices.
I've just searched eBay for BMW R100 and R80; 1 bike for sale?????
"I think" you are sort of right... Airheads and old Jap aircooled dinosaurs are in vogue at the mo as people can buy them for some dreamy café racer/bobber/retro project like a million other people... The bikes always age in order so good bikes of a certain generation will become somewhat forgotten until they become "classic" enough to be in vogue... See Fireblades, CBR's, oilheads etc... They will be less popular than some peripheral old crap from the previous generation as people sweep up all said old crap to be on some fashion band wagon...
What is a concern as chops and bobbers go out of vogue is that some really good old bikes will be no more as people make some bollocks job of a solid bike... See streetfighter/ 70's chops etc

and the VW "classic" car market...
Markets are always changing in a subtle or not so subtle way... Bikes are no different... When I couriered in London in the late 80's and early 90's the yuppie types changed fashion every few months and in a two year period had done jap sports bikes, Harleys, Italian classics and old Brit iron... All in vogue and then out of it... At the same time was the ridiculous market in classic cars that blew up in everyone's faces...
In economics the rough "bubble" theory in that approx 75% of the cash gets put in after the "bubble" has peaked... This is probably pretty accurate for the present fashion in bikes... Remember that several years ago it was the Charlie Boredoms of this world that were on about their wonderful bobber's etc... What also seems the case is that far more work is often done to bikes that riding them and that is down to "whatever floats yer boat"... Obviously this also involves loads and loads of chatting over lattachino's...
What is a big falsie is that you buy an old knacker for £2k, pull of half the useful bits, make a semi fist of throwing a couple of £k in "project" bits at it so making it more shiny and blingy but probably far less rideable and then trying to sell it just finished or "nearly finished" for £4k... Now that is bollocks... Good jobs on good bikes will usually be worth it but there are far too many over inflated half arsed jobs out there as people try to make the cash back for their flights of fancy...
As for old Beemers... The fact that the amount produced was far, far less than they produce now so creating a certain mystique and the fact that they hold together reasonably well makes them potentially stupidly over inflated... The fact that a decent airhead GS is about twice the price of a reasonable oilhead or that an airhead RT is on the market for a fair bit more than an oilhead RT is crazy... Even K's are getting over inflated now and they are arguably far better bikes for build quality than many an airhead knacker...
PS Even 10+ years ago working at a dealer, we steered a wide berth around airheads and older K's as the owners value and expectations was far higher than their cold market value... And selling bikes you can't bugger about waiting to convince someone of the "classic" value... There are also plenty of "traders" who have jumped on the bandwagon hoping to cash in so inflating the whole situation... See eBlag...
Watching the market evolve is interesting but you do notice the same trends come around again... It may even be in a year or two people get sick of riding oversized oiltankers called "adventure sports" bikes...
