That's not too difficult to imagine. Noa's arc, Hilter's ubermensch...

Keep some of the brightest and beautiful, replace the working class by a robot workforce and then get simply get rid of the rest. With the right nudge, these are quite capable of killing themselves off by the masses.

5-10 mln. people should suffice then to run this planet and still have an interesting enough life on it. And face its next new problem: how to deal with eternal life:)