Alright, so I think we need to agree on the nature of the time travel we're talking about in this. If we don't have a standardized understanding of how it works, things could get very confused very quickly.
When we travel back in time, are we creating a branching timeline? Or are we messing with a single, continuous timeline? The only reason I bring this up is that there have been a few mentions of ensuring that the characters are born even after defeating Darkseid. Whether or not they're born seems like the least of the problems posed by a continuous timeline.
Under that paradigm, if they were successful in preventing Darkseid's purge and subsequent enslavement of Earth's populace, they would effectively also be preventing the future Batman and Luthor from constructing the time portal that was
absolutely necessary to travel back and make that change to the time stream; it is a categorical paradox.
Not that comics don't have paradoxes when it comes to time travel, but it's not my favorite way of looking at it. The continuous timeline paradigm is fatalistic (i.e. any time travel that takes place logically ought to be observable in the first iteration. If no branching future occurs, whatever happens already takes time travel into account), the branching timeline paradigm is deterministic (i.e. going back effectively makes the future ambiguous, allowing actual changes to be made).
Singular Continuous Timeline:
Branching Timelines with Time Travel only going backwards in time:
I suppose that even with branching timelines a character might want to ensure that a version of them is born into the timeline they're currently inhabiting, but that version of them would be a separate individual. It's not like the character themselves would fade out of existence if they weren't born in the new timeline. They're already an out-of-place artifact of a separate world; they've been born already, somewhen else. If the events of the new timeline could change their pasts, not only shouldn't they be born, but they shouldn't have been able to come back to save anyone or change anything.
If I'm over analyzing this and making it un-fun, let me know and I'll stop thinking so much. I'll stop beating a dead horse now.