Having been active in the old and collector car hobby for many decades, I can tell you we watched this happen time after time.
We watched the desire for Model T Fords drop at a point that the last folks who drove them when they were still considered "fresh" quit driving or died. Bottom fell right out of the market.
I had a Model A Tudor, jointly with my brother. All done but the interior upholstery...Army blankets over the new springs.
We calculated the year the bottom would drop, based upon the T, and sold it (mechanical brakes and all) for $6500 at the car swap meet held annually hereabouts.
Next year, you could get a 31 A with juice brakes and overdrive for $3500.
All done, drive it anywhere at 65MPH.
Sure, it's come back up over the years as youngsters become enamored with old stuff....but usually, it seems, they buy them to put a TCI chassis, small block, 12V, A/C, IRS and all that into them.
They do drop....like stocks, you need to know when to sell, and when to buy back later on!
Dave