Hell, we only have to do it once per billionaire. Since they only got to that point through their own self sufficient blood sweat and toil they'll have no problems doing it again and better with all the experience they've learned along the way!
If we always kill off 99.9% and grant the remaining 0.1% who actually bounce back immunity, we'd end up creating an even worse billionaire class. Kinda like with antibiotics.
No matter how many times you let that happen to a bacteria, as long as you're killing them with a handgun, it keeps working. They don't evolve defenses without becoming something different.
So I guess it's a bit metaphorgotten, but the point is that that's only true if it's a system where starting from zero to a billion is something that can be selected for, and it pretty much just is not.
I mean, go for it? I'm sure the army would welcome the opportunity to invest in effective armor research, so if you can get it off the ground at all contact DARPA.
