They already did kill two big ones. Neal and Graham. Remember?
This show hasn’t had the stones to kill off a major character (Rumple, Snow, Charming, Belle, Regina, etc.) since Neal. Neal was huge but since then, nothing. And I’m not counting villains of the arc deaths like Cruella and Ingrid and Pan. I just don’t see them killing any of the major ones. Robin is big because it affects Regina. If Regina wasn’t dating him, he’d be useless. To me, Robin makes the most sense from what we have seen and heard from rumors. But as RG put it, whoever dies could come back because of the lack of stones this show truly has had in the past with characters coming back (Blue Fairy, Pinocchio, Zelena, Charming when Snow somehow shared a heart with him, Archie, inevitably Hook, and probably a few more I can’t think of right now). Although you are right, they did kill Graham which was big when it was done, they did kill Neal, and they did kill Merlin so anything is possible. I just have lost faith in them being able to stomach killing a major player until we finally reach the end of the show. And we are not there yet so I think Robin makes the most sense.