You have a valid argument, a laser could possibly be able to interfere with a shells flight path. Yet the problem with CWIS is that it relies on radar to detect and track incoming threats. While shells could possibly slow to the extent that they could be detected after flying for some distance, shells that are plunging or integrated with rocket boosting of some favor would gain speed, and therefore possibly be extremely are to eliminate. As for BrahMos, I have actually armed the design with the missile system for further ranged targets. My only fuss would be the price tag. It is arguable that a modified shell could reach a comparable price to BrahMos.
shells that have rocket boosting = an rocket/missile.
also, shells go about mach 3, IIRC..... assuming that there are anti-ship missiles that could go mach 2, I would assume that taking out an mach 3 projectile would be possible. detecting them would be absolutely no problem, unless you make them stealthy shaped and covered with RAM materials.... which would raise the cost to about that of an missile of similar size, especially when you count in the barrel life and cost.
you are right with the fact in mind that an phalanx could not stop an large shell, but missiles like ESSM and RAM will most likely be able to stop them before that.