IP is a bargain between state and author. The state agrees to let the author charge rent on their IP for a limited period of time, thus incentivizing the creation of that IP. In return, the public gets to enjoy that IP’s existence - and eventually enjoy it for free.
How long should the limit be? Here lies the debate. The longer the period, the greater the incentive to invest in production of the IP. For example, a movie studio spends $300m+ on a movie because they expect to gain that back via an income stream that lasts for many decades. If their period of protection was only, say, 5 years, their return on investment would be less and thus the investment amount would be less.
So one way to think about the copyright protection length is to ask: do we want to live in a world where investing $300m+ in a movie is a rational thing to do, or would we be content with movies costing no more than $30m, or $3m?
Sure, celebrities are paid too much, but for some variation of this argument you may agree that reduced copyright terms limit the type of movies that can be made.
You can substitute movies for pharmaceuticals if you like - longer patent terms mean a bigger payoff for investing in new medical research. Tweak patent length and you change what drugs exist.
I personally disagree with the patentability of some things (like software), and with the length of copyright, but I also like that movies like, say, Lord of the Rings exist, so it’s not a trivial issue for me. I want a healthy public domain but I also would be sad if the Bakshi LOTR version was all we ever had.