If Zoro dies as the WSS then the whole purpose of becoming WSS will die as well because Zoro won't be alive to defend his title against new rookies who will want the title just like how Zoro wanted it in the begining.
Agree with the bold. Also if you look at the scan below you can see that Shanks is pretty far away from his Ship. If time manipulation wasnt involved then Shanks either jumped spider man style or he ran that fast.