Do you have a link to this research?
The Western declined in popularity within 60 years. But there were popular Westerns made after that period (Silverado, Pale Rider, Unforgiven, the remake of 3:10 to Yuma). Father Brown was moved to the 50s... maybe it was cheaper to find props for that era?
Crouching Tiger, Hidden Dragon was pretty popular. So was Gladiator. Peaky Blinders? All quite a bit past that 60 year window. Not seeing the trend you're talking about.
Spotting a single common link might indicate a pattern, but it usually does not. That's my peer review of your article!