I'm not an old bike expert, my real love is old cars, but due to a sad set of circumstances and the resultant poverty, I can't afford the classic stable I would like.
But in the old classic car business there are two reasons for continuing availability of parts:
The value of the car - it is viable to have one off parts made if the result will be stupidly valuable, eg classic Bugatti.
There is enough of a following to remanufacture parts in relative bulk (or at least tool up for them, eg MGB
Or enough in the scrapper to salvage parts - unfortunatley the part you need is probably the part they were there for in the first place, eg any bit of body work on a 70/80's Italian anything.
I don't see enough widespread interest, or value, in bikes to keep that sort of support going, so older cheap bikes will slowly return to where ever LGF finds them, and a few really nice ones will increase stupidly in value and become ornaments.
I don't really care. I ride a (cheap basic) modern for the more serious stuff, a middle aged nothing special road bike, and a £34 retro off Ebay. I was going to upgrade everything and make it fancy, but it's nicely ratty, all original (except for an old Flexstem and more comfortable bars I had lying around) so I can't be bothered.
I enjoy the riding.
My 2c