I think your summing up is pretty well spot on Simon.
Watches (Rolex) and sports/collector cars both went silly at a similar time.

Demand was very high, the Rolex Daytona with the ceramic bezel caused a storm when it was released. Very low supply caused used prices to increase totally out of control. Everytime one was sold on it went for more and more money. The frenzy spread to all the other Rolex sports watches, and all prices went berserk. Some folk are now probably left holding watches they paid way too much for now the market has settled.
It was a similar story with the Porsche GT3 and GT3rs. Unless you bought several cars you didn't want you had no chance of getting a GT product.

I guess that not having to pay tax on profits fueled the frenzy.

A look at a dealer like Watchfinder tells a tale. They have a huge stock of watches which look very expensive in the current market.
That's a situation that is mirrored in the classic car world.

Morgans have always been, and will continue to be, niche. It's a small market and the 'traditional' buyers are getting older and aren't buying the cars any more. MMC's problem now is to try and find a new client base in a market which has all but stopped.
I noticed that another 'new' dealer has been taken on by the factory at Silverstone.
Good luck to them, if the 'legacy' dealers can't sell new cars I think the new dealer might have a hard job.....!