The biggest problem is linkrot. As a rough estimate 13% of links die every year, and it's quite possibly much higher than that. (https://www.gwern.net/Archiving%20URLs) Without the glorious web archive, bookmarks would be unusuable. And I wonder how many people know about web archive.. Youtube-dl may also be useful if you want to preserve music or videos (despite the name, it works on almost every site I've tried it on including audio sites.) Someday I intend to script something up to automatically scrape all my bookmarks and make a local copy, but it seems complex.