Well I could buy your argument that the usurping male kills the cubs to prevent future challenges if I could find evidence that male lions will copulate with their mothers and aunts and that the usurping male lions only kill male cubs.
Make that mother, aunts, and sisters. Male lions are upwardly mobile, and the greatest hazard for the head male is the younger males in his pride. Eventually, he’ll be killed by a younger male, who will take over his position.
Pretty sure they try to kill all cubs. The female won’t go into estrus if she’s still taking care of other cubs.
Looks like someone’s a cat person.