I'm gonna go ahead and say SPOILER AHEAD as well.
As far as I'm aware of, they never said the summons were gone for good. The summons are physical representations of the fayth. But there are multiple fayths. We just only see the bahamut one in game.
The only reason they go away at the end is due to them having met their goal of stopping the sin cycle. If a fayth has any motivation to come back, I don't see why they can't.
I mean we had anima be made out of Seymour's mother (IIRC) and he clearly had 0 motivation to beat sin. So that could imply there's fayths who have a different agenda.
