You said it right: "aren't VERY maneuverable" and not "UN-maneuverable". They can still maneuver. Secondly, because a long range CM is capable of Supersonic / Hypersonic speed, does not mean they will be flying at full speed all the way. In order to effectively get to its target, it will vary it's speed to effectively maneuver.
The range I had in mind was 5kms~ 10kms. At the range you mentioned, the chances of missing the incoming missile are even higher. It's a missile you are targeting, not a projectile flying straight like an arrow.
Obviously, it was given as an example as to why, and how a dumb rocket (as in without a seeker) could fail.
2nd Strike, 3rd Strike, MAD etc is a different story. We are discussing defeating an incoming missile at the moment, and how an MLRS system could be used as CIWS for it, as a last resort. So lets stay on topic.
In your opinion, what would be the probability of "100 dumb rockets, like you said" taking out a Brahmos category CM?
Well easy or not, it is not an option, and has to be done.