I think as whites go down in population the power of JBW will increase exponentially. Though, odds are the whites that are left aren't going to be tall aryan chads, as those will be the ones who are targeted first by ethnicels out of envy. When the population decreases to the point of about 10,000 or so people, any chads left in that group would probably be able to fuck any ethnic female on earth. The white foids are going to fuck the ethnics even if they are ugly, because women always fuck the conquerors in an effort to preserve their genetics, but because blonde hair and blue eyes are so rare at that point, seeing a chad with that combo would probably make any ethnic foid go wild. Noodlewhores and curryfoids would be the easiest prey for them, as both cultures seem to obsess over the aryan chad.
Any whitecel in here is gonna be fucked though, aryan Chad's gonna be too busy fucking everyone else that we won't even have a single chance at reproducing outside of forced impregnation. Though, abortion is a simple fix.
TL;DR White's dying out will increase Ubermensch Chad's power level and cripple what little power we whitecels have left.