I think some Morgans ARE rising in value

Just look at +8 prices, early ones are now very much sought after, I would even suggest that the last of (this) breed have risen (quite steeply) i.e. since 2016.
I say this as a last model +8 was my first choice when I started looking for a Morgan, in a serious way.
In early 2016 several +8's, circa 1998-2002 age range, could be purchased for £30k....those cars are certainly (in demand) and now command £33-36K for the same car.