Yoshida has repeatedly stated in interviews that the story will end in 6.0. The ending narration of 5.3 straight up says that 6.0 is going to be the final chapter.
Ofcourse, FFXIV is profitable so we will still get more content (probably set in either Meryacdia, the New World or (5.0 spoiler) the original timeline, but the story of the Ascians, Hydaelyn, Zodiark, and the sound will end in 6.0.
- - - Updated - - -
See above