[Tens of billions? Knives and Vash were from a very far off future, Earth in his time hadn't even reached ten billion yet.] Since you're so familiar with them, you'd know the last thing they'd do is kill off their own in an attempt to control the population. That only happens when they decide to have a petty war between themselves. I don't think that's something that will ever change. So it only makes sense for them to look for another planet to survive on.

[He wonders what's got Knives so disgusted with humans. He seems to like them less than Vegeta does, and he's never had a very high view of them himself.]

The weak are just as useless. I've only found one particular instance where they were useful.