The long timelines stifle new creative works by keeping other, especially smaller, outfits having to make sure they don't accidentally run afoul of another copyright from decades ago. This needs capital to either be proactive in searching or to defend a suit that's brought.
Here's a recent article about the battle between the copyright holders of Let's Get It On and Ed Sheeran for Thinking Out Loud. Those two songs are separated by around 40 years. https://www.theguardian.com/music/2023/may/07/ed-sheeran-cop...