1958 was actually a recession year, the Eisenhower Recession it was called. Sales were below average for all makers. The 1958 Thunderbird, Robert McNamara's brainchild, was the only model that saw a year of over year increase.
Those huge luxury land boats saw dismal sales. Only a few thousand at best. One would think the limited number would make them highly valuable among collectors. Rarity is a factor, but other factors affect collector value.
Convertibles have never really caught on in America. The British makers imported those little two-seaters going back to the 1930s. Chevrolet's Corvette sale didn't hit 10,000 until after 1960.
I own shares of other companies, most of which were purchased in 2009 and 2010.