BravoBuds
Crocodile would not be able to get Pluton, because Pluton is not in Alabaster. Its in the new world, disguised and heavily guarded by an army of minks. An even if he did manage to somehow(I highly doubt it btw) get to Zuneisha, and somehow(not gonna happen in a million years) defeat the minks. Its not going to listen to Crocodile at all. But you make a fair assesment. Without Nami or Robin, accomplishing Luffy's dream would be next to impossible. Because only Robin knows how to read the poneglyphs. An if she dies without anyone to be taught said language, then yea the world is screwed.
I don’t see how a new krieg (maybe the new krieg would be Gin?) wouldn’t pop up. After the ‘angle pirates’ thing it should have shown the pure power of having a huge pirate force that could deter the marines. Probably the only character which would be harder for Luffy to get is Nami, but you could write it off to have her crew abandon the pirate life once Nami got a bounty… so Nami then can’t go back to her village, but has no crew, so joins Luffy.