My answer has no spoilers as I am just going to be referencing theory. I don't think there is any way for multiple universes to ever not exist. If I went back in time to do something that creates another universe, that in the end effects the future of my universe, then how can that universe not exist anymore while still having the results of my changed universe.

I think that it would be impossible for the other universe to ever not exist or else the thing that came from that universe that changed my universe would not exist and therefore my universe would not be changed. I tend to lean towards the infinite universes where every single thing is possible. There is a universe for ever situation. None get created/deleted, just everything exists all at once all the time.