The Russians use what they use because it's what they have and what they can afford. If they had better, they would use it. As for this new missile, from what I understand it gets its speed from the re-entry into the atmosphere which every nuclear warhead has, it has no internal engine to increase speed. What makes it different, if real, is that it can maneuver once deployed. In other words, it is gliding to the earth at hypersonic speeds and it can alter it's course to avoid missile defense systems and then re-acquire it's target. Sound like something the Russians would be able to develop before the US? While possible, I doubt it. Solution if real is to destroy the missiles before they deploy in the first place. Star Wars which the ridiculously named Space Force will take care of. I can't help but visualize Space Balls every time I hear that name mentioned for some reason or another. Thought it was a joke but apparently not.
A good example of this development is the US spent tons of money developing a pen that would work in zero G. They did it. One American astronaut supposedly asked one of his Sovietcounterparts how they solved this problem and the answer was "we use pencils" Sometimes more advanced isn't needed when you get right down to it but sometimes it is.