I've always felt that it should be the lifetime of the creator and if there's no heir established to carry the reigns of the IP - then public domain. Companies like Disney love to lobby and push that number and have pushed that number to ridiculous lengths so that it's like 500 years (exaggerated since we know it's like 90+).
Because say the creator dies, they have not picked a heir or that they don't have anyone to entrust with, with their IP. It gets funneled through the state's laws of inestate succession.
However, what we mostly have seen is when an owner does give up the rights and it's in the hands of greedy mongrels like Disney. What happens is that the IP is just in a cycle of re-release hell just to keep whatever trademark or copyright alive, doesn't matter about the quality which is usually shit.
And in the video games industry, we've seen copyrights to games that will never see the light of day. Copyrights and IP rights get hot potato'ed all the time because there's so many people involved that it complicates things whenever a creator dies or vice versa. It's why we haven't seen digital releases for No One Lives Forever 1 and 2.
So, copyright has unnecessarily made things complex to where IPs are just used as extensive methods of profit, some of which aren't even being directed to the original creators. Which makes me feel like copyright should just be the lifetime of the creator and then outsourced to public domain.