Personally, I don't care who predicts what. Like others have said, 1) I didn't get into this sport to make money; 2) I have enough spare stuff already to last at least my lifetime; 3) There is still PLENTY of new old stock on eBay. If I don't want new old stock, I can still get brand new consumables like cables and brake shoes.
It's really a silly discussion, if you ask me. Real Chicken Little thinking. I help fire a 1941 steam locomotive on a local tourist line. I can still get vintage and operational steam locomotive equipment off eBay (and have done so). If you think old bikes are rare, trying finding parts for a 1941 steam engine. And yet, it's done ALL THE TIME.