As I recall the reason for possibly going past 4.5 was stated to be Return to Ivalice... I don't remember where, but I read that was why.

That said I don't expect the MSQ to go past 4.5(.5). There are a couple lingering issues (Fordola's path and the Zenos-possessed Elezen) that need to be dealt with, but that's doable within one patch. Afterwards there's just the matter of how we're supposed to get into Garlemald without the Warrior of Light's [in]famous reputation attracting too much attention (if you volunteer to go with Thancred on his scouting mission he shuts you down for that exact reason).

There are things that can be done past 4.5(.5) story (Omega could go past the third tier, Ivalice can have more than three parts, Kohryu could be the real final boss of the Four Lords series), but...