"Cannon shot, even if not terminating the walkers, would incapacitate them and/or slow them..."
Maybe, maybe not.
How many rounds did Shane pump into Herschel's example walker to show him that they weren't just sick, but dead?
That one wasn't particularly slowed...
Sure, if you have a large enough cannon to throw a shell or other projectile(s) large enough to have some sort of effect, it MIGHT be worth the expense in manpower, resources, time, inherent lack of portability, etc.
But sorry, I'm just not buying that there would be enough use out of a cannon to justify its disadvantages.
However, if you were to have yourself a static position, say, a fortified compound, then that's an entirely different story. Then a cannon could very well be useful against herds of walkers.
How about a Napoleon 12 pounder firing chain or bar shot?
THAT would be effective as hell, as long as you could figure out how to keep the chain from breaking (which was a big problem), and which is apparently why bar shot were ultimately developed.