What will you do or feel?
In any cases, if ever one of them have to leave the crew for him/her to protect an island (or let's say have to be left behind), i would really expect someone with an opposite personality would replace the member temporarily.
But if there are someone who is legitimate to be voluntarily get left behind it would be Nico Robin since she's a archeologist and loves to study things around her, finding a way to stop or to decypher a written history just to help them to save/find the World/One Piece.
I know, i know that it sounds impossible since OP will not be OP anymore without the other members of SH. but hey! Oda killed Ace and Whitebeard who has been favorited by many.