Bookmarks are perceptually longer-term than open tabs, so there may be more reluctance to save to a bookmark. (E.g., if planning a trip to Italy, do you want to bookmark some blogger's food recs for Rome, forever?)
But worse is, it relies on recalling the text in the bookmark's title to resurface it. You might not remember the page title, but you can always scan through open tabs.
This is an interesting idea for a feature, that I think I would like too. I like to save things to maybe look at later and a TTL would manage automatically dropping them from bookmarks in case I never actually want to look at it later.
I add a folder to my bookmark bar. All project related tabs get bookmarked there. When I'm done, I either delete the whole folder or file it somewhere.
But worse is, it relies on recalling the text in the bookmark's title to resurface it. You might not remember the page title, but you can always scan through open tabs.