If that is so why not, you know, just supplement S-400 with more S-400 batteries?It has been discussed that S-400 are upgraded modernized S-300. And per Russian doctrine that S-400 is to be supplemented by S-300 batteries for best effect. S-500 are again upgraded S-400 system with more ABM capabilities. Correct me if im wrong.
I think the Russians just decided that S-300 is good enough system for majority of things so even if S-400 is available they are still gonna keep using S-300 they already have.