"Destiny" is nothing more than knowing the future and seeing how things turned out. If someone "has" to be there for an important event, it's not because it was preordained by some cosmic force; it's just because they happened to be in the right place at the right time for that particular timeline to turn out in a positive way. That's what happened here. In the original timeline, Janeway happened to be in the right place at the right time to make the key difference. When "Endgame" altered that history, it led to a timeline in which Janeway wouldn't be there to make that key difference, so Q Junior had to correct that.