I'm convinced that a significant proportion of people today don't use bookmarks and have never leveraged their power because they grew up with tabs and never bothered to explore their other options for organizing websites.

You can save all of the tabs of your current session as a bookmark folder in one fell swoop! Your research tabs can be all saved together and opened as a group! Your gift ideas that you won't close because you don't want to forget about them can be saved in a folder named gift ideas so the next time you need them you have them, without the cost of using up your extra RAM and CPU cycles!

On modern browsers, don't unused/old tabs take up basically no resources?

I remember both Chrome and FF making a big deal over that point years ago.

There were extensions for it more than a decade ago already, but these days it is indeed the regular out-of-box behavior (though Chrome got it only a few months ago IIRC).

Nah, I grew up with bookmarks and was constantly annoyed at the terrible UI for them and then when tabs got good enough I stopped using bookmarks since the default bookmarks UI is terrible.