The case the movie seems to make (through Neil) is that it's unknowable what would actually happen, but that people in the future (the ones directing Andrei Sator, at least) believe that it would work out for them (or are willing to risk it).
The classic thought experiment – if you went back in time and
killed your own grandfather, how could you have been born to commit the
What’s the answer?
There is no answer, it’s a paradox. But in the future, those in
power clearly believe that you can kick Grandpa down the stairs, gouge
his eyes and slit his throat without consequence.
Could they be right?
It doesn’t matter. They believe it, so they’re willing to
If the future wipe out the past where their own ancestors live, then they can't exist, but if they don't exist then they didn't wipe out the past. There is no consistent explanation of how this could work (without invoking multiple timelines, or some sort of "meta-time" in which the regular timeline can undergo change; the characters have no evidence of any of that, and arguably couldn't observe any evidence even if it were real).
It's possible that the timeline can in fact undergo change (or fork multiverses), and so can change from one the one we know to one in which people from the future are living backwards into a completely different past. Perhaps in the future there is some scientific evidence of this, or perhaps they just take it on blind hubristic faith.
Or it's possible that the attempt to eradicate the past will eradicate both the past and the future.
Alternatively, it's possible that (as the protagonist wonders immediately after the quote above) the mere fact that the characters exist proves that the algorithm will never be activated.
The movie does not attempt to resolve this question. Rather it simply posits that the people in the future directing the war have resolved the question for themselves (whether in truth, in error, or in hubris, we do not know), and have determined to try to eradicate the past.
The protagonists believe that they need to fight against this effort, even though they aren't sure whether their enemy can actually achieve their goals; they aren't completely sure that the enemy can't win, either.
And thus we have the main conflict of the movie, even though the movie makes an argument that there is nothing to fight over.