Free agency changed the landscape for sure, but it didn't all happen at once in the 1970s. Big markets stockpiled talent, but small markets were still able to compete -- Cincinnati was actually right at the top in attendance most of the decade, behind only LA. Cincy, Baltimore, Pittsburgh, Oakland all won titles after the mid-70s. The Pirates had good clubs in the early 90s, but after Bonds left never finished first again.
The Bucs, Reds, A's and O's made it to 12 World Series in the '70s, five in the '80s, and zero since... in 32 years.