There are several songs i play that have some sustained notes that play over several bars and i never think too much about them.
That’s an interesting point! I saw a bass player kind of brush over the string to keep it going without plucking again.
And from singing, I can tell you that more sustain would lovely. Guess I have to buy a high mass belt buckle or something. 
I thinks “more sustain” is somewhat relative. Many things can boost natural sustain but not really noticeable in most situation. If your bass has naturally short sustain like the Beatle violin bass no amount of mass or better yet saddleless bridge would give you noticeable super sustain time.
Usually it’s not the presence of something but the lack they’re off, that catch your attention. I came across some Squier basses that had the plastic neck plate guard installed. Some are almost like hard rubber. The playing sensation is a bit different, it felt dull, the notes felt sticky. Removed the piece, it felt “normal” again.