Dean, having a foot in the classic market, a lot of what you say is true, in the bike world generally there is a downward trend, the only areas bucking the trend seem to be military WW2 machines, 1970's Japanese and Italian machines and top of the range limited edition bikes from 1990/2000's ( Ducati especially)
The age demographic of the owner now has sealed the fate of many 1940/50/60's British lumps of iron and the forty year old rule for free tax and non-requirement of MOT has driven up the desire for just pre 1984 machines, this is largely why, as you say, the rubbish we were buying for a few hundred quid in the day as a second hand vehicle have now become so desirable in the car world, this includes a lot of what were absolute shockers in the day , allegro, avenger , ital ,marina, Princess, Granada etc. meanwhile anything with a low survival rate from this period like alfasud , Lancia Beta, Saxo etc that rotted to death if they went near a puddle are also now desirable, before we even get onto Escorts, capris,205's Golfs etc
Meanwhile, like the bikes, the older cars are in a downward spiral, I have sheds full of bikes which most are now not worth the price I paid for them when I bought them, for example I have a 1966 Triumph TR6 Trophy I bought about 6 years ago for £7k, I doubt even on a good day it would be unlikely to achieve £5k now.