What gets me is if there's a constant crush of zeds against the fence, and Maggie is sitting there for hours stabbing heads... where do all the dead bodies go?
Stab. Stab. Stab. Stab. There's 4 corpses. All the walkers should now be trampling on top of those corpses, and the next row should be six inches to a foot higher. A mound of bodies would be there very shortly. The walkers climb the bodies, but Maggie or other stabbers have nothing to climb to get to the elevated targets.
Granted, the walkers want to get to her, and not necessarily "inside." So if she walks sideways the herd will follow her to another low spot. But doing that for an hour is going to result in hundreds of bodies on the ground. Eventually, the whole fence is going to be lined by downed walkers. Especially if the problem is as bad as it was presented.
Frankly, I think that was too many walkers at too close of a range for Rick and Carl to survive, even with M4's. Must have been a couple hundred walkers that pushed down the fence.