I'm not defending what AMC did to The Rifleman, but I'll defend the network in general.
First, the majority of the audience out there - especially the audience advertisers want - do not want to watch classic films from the 40s and 50s. Even 60s films must seem ancient and dated by 30 year old viewers who weren't born yet. I used to run video rental stores, and I can tell you that even back in the 80s - the Classics sections of our stores went mostly untouched. Only a handful of old films (Casablanca, Public Enemy, Gone With the Wind) ever rented. The rest just sat on the shelves and gathered dust.
Second, most of these cable networks have gone generic, loaded up with true crime stories, off network reruns, and reality shows. At least AMC has bankrolled some excellent original series - like Mad Men, Rubicon, Breaking Bad, and now The Killing. To me, AMC is light years ahead of other disappointing cable channels that have abandoned their original purpose - like Bravo, and A&E.