I didn't say it wasn't useful.
If your ABM systems are not based directly on the target then being able to intercept to 200km means you can intercept a ballistic missile bound for another target.
the very high flight speed means that even a small control surface will be effective in directing the missile in flight, but as you get higher and the air gets thinner then those small control surfaces start to have less effect.
They keep them small to minimise drag and weight, so making them bigger would be a bad thing and would not extend the effective altitude that much.
Side thruster rockets would be used in the terminal phase to adjust the path of the interceptor missile rapidly enough to make a difference, but in a near vacuum there is little air flowing over the missile body so a side thruster could just as easily start the missile tumbling which would obviously present a problem if the seeker in the nose can't see the incoming target.
The ISS operates at about 300-400km and of course it is not alone so being able to hit ballistic missiles at 200km is getting very close to being able to easily hit stuff in orbit.
I suspect the S-300P and S-300V and S-400 series got a limit of 30 km is because they probaby found that to be the right balance for air breathing targets and BM.
Like I said, the S-300V was developed in the 1970s and hitting satellites would have been counter productive for the Russian Army.
The S-400 is based on the S-300 and therefore has its features and limitations as well, except in this case they are not really limitations.
Complaining that the S-300 and S-400 can't hit targets above 30km is like complaining that my assault rifle can't reliably hit targets at 1,000m.
S-300 and S-400 are air defence missiles that defend potential targets from everything from cruise missiles and high flying supersonic recon aircraft to ballistic missiles.
If you want a dedicated ABM system that only shoots down BMs then that is S-500, which will be almost of no use to the Army or Navy or Air Force, except defending strategic assets like ports and cities and major airfields and strategic weapon sites.
S-500 is a SV-338 and will likely be accurate to 1,500m but it is no substitute for an assault rifle.