There has been no reason to expect an Indy 6. That has purely been wishful thinking on the fans' part thus far. What I've gotten from Disney's statements to date is that Indy 5 is intended as Ford's finale, and that it will then be followed up with a reboot of some sort. How the fifth film will connect with or set up the reboot (if at all) remains mysterious.
That said, I don't think Disney would be opposed to a sixth movie with Ford, and I think Ford would be game too, but only with Spielberg. And considering Spielberg allowed an 11-year gap between the fourth and fifth films, I don't get the impression he'd put all his other projects aside for the quick turnaround the project would require. It would have to be pretty much back-to-back to work, I think. Spielberg is 70, and would probably not prioritize a sixth installment of anything over other projects he'd like to do. Maybe he'll surprise us though.
Five movies feels appropriate to me, and ties in nicely with the original five-picture deal Lucas made with Paramount. I'm sure that's been moot for decades, but still, kind of poetic. Six movies would give us an Old Indy trilogy to go with the original trilogy, but screw symmetry.