The world ending really don't effect the SH goal .
If anything them finding the OP is what going to help them take control of the world and help stop it .
I feel like it should be somewhat of concern to them that the sea levels are going to be rising so heavily that all the islands they've visited, with the friends they made along the way, are going to flood and be uninhabitable. Goodbye to the homes of all the straw hats who had one.
You're not gonna tell me that members like Nami, Sanji, Usopp and Robin literally don't care about the idea of an ancient weapon just being used to destroy islands on a dime and flood existing ones.
And as far as "finding the OP" is concerned, they are literally just hanging around exploring Elbaf for no reason other than, "it seems like fun".