I cant imagine those shows lasting much longer than they did past 1970, purge or not. Two years tops for some (such as Mayberry RFD, which really ceased to be after Griffith and Knotts left). Even Gunsmoke, which survived the initial purge, was running on fumes by the time it was cancelled in 1975. Cancelling Hee Haw was dumb, since it had barely gotten started and ended up being a huge hit in syndication.
Even so, the rapid social decline was already occurring even as those shows still aired (by 1965).
I think it was the canceling of the Westerns (too violent) that was an even bigger factor.