First they ignore you, then they laugh at you but when they start fighting you, you know you've won.
My first thought is that people running Mastodon wouldn't qualify as data controllers, but I am no expert in GDPR laws.
Here is a chart of CD sales
https://www.statista.com/chart/amp/12950/cd-sales-in-the-us/
You do not see any sharp drop off because of Napster.