While true in some cases and perhaps in this one, there are a number of cases where unpopular bikes become collectable, notably the 888 and early year BMW R75 among many others. The 888 was around for 2-3 years and didn't sell, mainly because of 916 rumors. Many 888's were sent back to Italy and the frames that Ducati had produced were used for the Monster. The R75 came out in '71 with chrome side panels on the tank and the "faithful" hated the thing as being to garish - essentially not boring enough for them. Both of these bikes were unpopular and didn't sell in the years they were available and are now collectable.
It is true that some things don't sell well in period, but as time marches on they become appreciated more.
Those tend to fall into the "ahead of their time" category.
I think that is not the normal outcome though.
The other thing to deal with in regards to rare, is the actual size of the market.
When Ferrari makes 300 of some special car, but there are 1,000 people in the world with the ability to buy it.....the prices stay strong.
It is both rare, and something with a demand.
Morgan does not make a tone of cars each year, but there really isn't a huge demand for them. So prices do not go up at all.
They are rare, but supply/demand is about equal.
Same thing holds true in the "vintage" motorcycles.
I have owned some very rare bikes, but some have been so special and rare that there are less people that care about them then bikes built.
So for that it was rare, but demand was not there.
Most limited production bikes still depreciate as they end up just being seen as "used" for a while.
That is what always made me wonder about sport classics.
They were not rare.
They didnt have demand when new.