Swizzling is not the only thing that would break. You'd break categories in dynamically loaded frameworks or bundles, and yes, you'd still break KVO because it

Swizzling is not the only thing that would break. You'd break categories in dynamically loaded frameworks or bundles, and yes, you'd still break KVO because it replaces -dealloc and -class on the newly-created classes. You'd also break dynamically adding methods to classes.