First of all, calm down.
Tabata backpedaled regarding the linearity of the second half of the game. It is actually towards the end of the second half of the game that the approach will become more linear/story focused.
Also, we still don't know what that more linear approach really means. I very much doubt it will be just FFXIII corridors.
You also will be able to fast travel back to the open-world areas at any time, according to Tabata.
There will be a second continent called Terra Verde that is still mostly under wraps. All we have seen of it is concept art and it suggests it is just as varied and open as the Lucian continent. Probably even larger.
Since the complete region of Duscae is, according to the developers, just 10 percent of the whole game world, that means that Lucis is about 30% of the whole game world and that there is a whopping 70% of gameworld left.
What suggests they must be past chapter 5?