I think Brian is quite right. The end is far from nigh. There is an issue at the moment maybe, as there are too many bikes in too few hands (and yes, too few of them do any decent mileage for all sorts of reasons, some good, some bad). See Olev's earlier point.
But this is an owners' age / baby boomer / affluent generation thing. The cycle will change as we thin out, and younger people will no doubt gradually acquire one then two then more bikes and the pattern will repeat. Not in exactly the same shape probably, because we slaver over the machines we had when we were young - which can't apply to twenty-somethings today. Nevertheless, any classic machine in one bit today can be presumed to be a heritage asset that will be with us for ever. That is starting to include all sorts of things we derided in our youth, some of which are actually really very good. I think it's nice to see a good early Honda 4, or Kawa triple, or any of the other iconic machines that took over from the broken UK industry.
All toys find owners, so I don't think there'll be a problem at all - there'll just be this cyclical bunching, then post-mortem redistributions, then the same again as the next lot takes over. Prices will fluctuate naturally according to the rules of supply and demand, but basic value will be retained. The overall trend will be upwards, I'll bet. We're all addicts, and the next lot will be - are indeed - the same. I have a daughter with the same disease, it's just less advanced!