I think the problem is that people generally don’t know when they’ll return to a tab, so they leave it open, and that creating a bookmark is asking too much commitment for something potentially ephemeral.
A good system might be one that doesn’t automatically close tabs, as some extensions do, but collapses ones that have remained unused for a certain period into a set of temporary bookmarks. Recently accessed tabs stay in the bar, while stale ones are swept into an “old tabs” list that can be perused at the user’s convenience.