I have in my collection a number of 1950s "silver" lenses, and they all have a high number of iris blades: 10, some even 12.
On the other hand lenses from the end of manual focus era (late 1970s to early 1980s) tend to have much fewer iris blades; often just five or six.
This holds true even for a lens that kept its optical formula unchanged over the years, such as the long lived Carl Zeiss Jena Tessar 2.8/50mm. It went from 12 blades to 5.
Manufacturing cost was certainly a factor, but could not be the only one — the cost and complexity of having a lens with many blades was at first accepted, and then not.
What was the reason for going from many to few iris blades?
I realize this is a possible duplicate to Why do lens manufacturers produce lenses with few aperture blades? - but I am asking why did the many irised lenses go out of fashion, which I take as a different question.
To illustrate my point: these two lens are optically the same (but about 20 years apart in manufacture).