True but it certainly helps when you have your middles positioned on the correct side of the ruck providing the right amount of cover to your edges enabling your your 3 and 2 men to have confidence that their inside is protected.Umm....
On a lateral sweep, two things, and minor variations of those two things, contribute to the effectiveness of edge defence;
i) Can the sliding defence mitigate the speed of the half/fullback's speed to outside of his man?
Or
ii) Can an up and in defence kink the sweep through jamming, by shutting down the half (early jam) or centre /fb (late jam)?
A collapsing middle can lead to a number shortage out wide, but this is generally accumulative over a set, rather than a measure of effectiveness of an edge for a particular play.