I am struggling to understand your argument here.
A similar US production boat of that era would be the Robert Finch designed Islander 30. It was a comfy and fast cruiser/racer. I raced many miles on one myself. The big difference between the Arpege and the Islander 30 was the I-30 was a more advanced design with a broad stern and a spade rudder. ..
You mean that the Islander 30 was the mass production cruiser more sold in the US on the late 60's?
"The ISLANDER 30 is a raised deck version of the ISLANDER 29"
ISLANDER 30 sailboat specifications and details on sailboatdata.com
The 1968 Islander 29/30 of that era was designed by Joseph McGlasson or at least is the information here:
ISLANDER 29 sailboat specifications and details on sailboatdata.com
It looks "old" when compared with the Arpege:
I think the Islander 30 you refer (by Robert Finch) was a 1970 boat designed 4 years later. They build the boat from 1966 to 1985 (19 years) and even so they only built 500 sailboats. are you sure this was the best sailing boat in the US at the time?
The Arpege was built from 1966 to 1976 and they built 1500 but the sales were residual after 1973 because the boat was replaced by the Dufour 31, a more modern and fast boat, a boat very similar to the Islander 30 Bahama by Robert Finch (1973).
ISLANDER BAHAMA 30 sailboat specifications and details on sailboatdata.com
I don't know if this one was the best seller but certainly sold more boats at that time: From 1965 to 1969 (only 4 years) sold 304 boats, the Columbia 29 MKII:
and then substituted by this one (Columbia 30) that from 1971 to 1973 sold 287 boats.
This means a total of 591 sailboats sold in 6 years, much more than the 500 islanders sold in 19 years.
Remember we are talking about main market and what defines main market is the number of boats sold: Those are the boats the average cruiser want, not the ones that want them to race. The Arpege was a main market boat, the Islander not. it seems the Columbia were much more but I don't know the American market of the end of the 60's and I don't know if there was boats with more sales (30ft) then the Columbia.