While it sounds nice (from a consumer's perspective) to have no copyright at all, it would prevent a lot of media from being made, e.g. expensive movies.
Movies for well-known franchises could possibly be financed through crowd-funding, but that would make it hard for production companies to make bets in new franchises or directors.
If copyright were shortened to 20 years as suggested, it would be legal to freely distribute copies of Windows 98 and Photoshop 5.0. I doubt that would make a dent in sales of Windows 10 or Creative Cloud. Newer versions of both would still be copyrighted, and there would be no obligation to release any unpublished source code.