User blog comment:Kaido King of the Beasts/Chapter 818: Stepping Stones to One Piece/@comment-7606564-20160303181833/@comment-10543621-20160304124701

That's if they succeeed in forcing her to read the poneglyphs. And even if they do, neither of them has all four. Unless somebody that has all four kidnaps Robin, she'll still need to be kept alive until they're all gathered. Besides, the 4 road poneglyphs aren't everything. There's still the final poneglyph on Raftel that apparently super important. They'll still need her for that.

In other words, Robin has a guaranteed trip to Raftel regardless of who become pirate king.