the market for family films has always been there. whether it was exploited or not was the choice of the film studios. The 1970s were filled with the movies that you mentioned because the filmmakers were finally free from the restrictions of the hays code and decided they would produce movies that pushed the envelope, for the first time, simply because they could. Only a few studios realized that there still was a market in family oriented content, Disney being the major, if not only, one. At the same time they also realized that they needed to produce those other films that appealed to a different audience, and began to invest in other film companies and distribution channels that weren't publicly affiliated with disney, so as to not harm their "family oriented" public persona. Miramax made Disney a ton of money in the mid to late 90s with movies such as the Scream Trilogy.
During the 70's nobody was lining up around the block to see "The Apple Dumpling Gang Rides Again". But the Disney name was so branded as "family market" that they were trapped. Hollywood studios nearly went bankrupt chasing the family market with big budget musical flops.