On the plus side is 356 scarcity. Newest ones are 46 years old! I don't see a big drop off for them. Certainly not the Speedsters or Carerras.. maybe a B notchback though
I also imagine in a similar vein to the Cobras, since the values may not fall, you might see a rise in demand for repro/kit versions.. Spending 25K on a kit vs 100K for a genuine speedster? sign me up!
Sure model T prices have dropped, but there were over 15 MILLION of them produced.. and Model T prices are likely always less than Model A prices (a nicer car arguably).