That can be a driving factor but even if they copyright expires publishing of said materials can still continue. There are plenty of free domain books that get sold by major publishers that make money. It's not like the money train has to stop once copyright has expired. Live performances of music and so on can generate lots of income.
The way that releasing copyright in a reasonable amount of time can help creativity is that people could use the beat track and write new lyrics. They could pull out samples and work them into other derivative works as well. Full covers could be done in a different tempo and style that would be new and exciting. As an example https://youtu.be/m3lF2qEA2cw it's the same but totally different at the same time.
Locking in copyright over a long period of time can stifle creativity.
The problem with a short copyright on music is this though. You have a new and upcoming band who hasn't had a breakthrough. They produce an awesome song or sound and it's just not caught on yet. Some major label finds it after it's gone past it's copyright and hands it over to one of the artists they have commissioned they produce it and make millions while the original creator of the content is stuck with a thumb up their nose wondering why the hell do they even bother. In that sense a low copyright on music and other materials can be harmful to the creator. Cutting a good balance is hard, but 70 years is way too long and so is 50 for that matter. Something that would give a 20 year old a good shot would be nice, say 10-20 years somewhere in there. That would be good for everyone. New artists could use the material that basically influenced them as a kid and the artists music has usually ran it's course by then. 3 years is too short.