I enjoyed your analysis. I think people should really just be putting 's on the end, then it works for all: 40's, 50's, 2000's. The apostrophe is unecesary,

I enjoyed your analysis. I think people should really just be putting 's on the end, then it works for all: 40's, 50's, 2000's. The apostrophe is unecesary, but IMO without it it looks like the 's' is a standard unit of measurement: 40s, 50s, 2000s