R’ Moshe sort of makes that argument and says if 3 million people live in a square mil it is a reshus harabim.
Though note there isn’t some chazal that says the 600k rule. Rashi says it and it isn’t clear where he got it from. Of course Rashi is broad enough for us to rely on. But it isn’t like it’s an absolute limud of learning 600k from the midbar, it’s a big enough chiddush in of itself we can’t really stretch the chidush much further