Every now and then, I declare tab bankruptcy, mass bookmark them (to get over the feeling that I'll be closing something important), and close them all.
I've never, ever, once, in 15ish years of operating this way, looked at any of the bookmarks.
[EDIT] I guess the main issue is that deciding to close tabs I'm not currently looking at takes time, because I have to evaluate each one, and when I'm down to just favicons on the tab itself, that means actually looking at each page. Just periodically mass-bookmarking and closing is less work. It's a UI issue. Plus, if I'm looking at my browser, it's because I'm doing something, and that something is basically never "playing tab-gardener". My very first action is gonna be "new tab" and go from there.