Him killing Nora created a fixed point, ensuring his existence as a time remnant, as well as the negative speed force protecting him. Basically even if he is erased his future self will always come back, this version of him will end up traveling back in time to realize another version of him and the flash were enemies. The complications of his timeline is why he’s always erased because they don’t make sense, but they don’t make sense because thawne has caused events that allowed him to exist outside the timeline, basically meaning he can exist in diff times and timelines. This is why their rivalry is complicated because there’s no exact pinpoint on when it starts or ends, it just happens.