Was the dip after 1979 the result of internet pirates too?
Well it was likely due to a couple factors;
1. The economic downturn
2. The lack of good music at that time (it sucked)
3. Cassettes. People were able to copy things for each other and play them in their Walkman, which was the disruptive technology of its day.
4. Cassettes. These had a lower profit margin than records because records could be pressed and cassettes had to be copied serially. They also had a lower selling price quite often.
17 U.S.C. § 1008 made that actually 100% legal, and it remains so to this day.
Audio Home Recording Act of 1991 mandated that if you bought a consumer audio recorder or consumer audio media, you would automatically pay a fee that goes to the RIAA no matter what your intended use of the devices, and also that consumer recorder should respect a "Do Not Copy" flag in the media and restrict media to be used to that which has had the fee applied to it. This broke down with the advent of consumer computer recorders which got around the prohibition by being data devices.
So, 1979 was a downturn, but legally the reasons were absolutely not piracy. What was happening was that consumers were exercising their rights under the law.
But no matter the reasons, artists get screwed in good times and bad by crooked record labels.
Until that changes, the RIAA isn't getting a lot of sympathy from ME.