Selection is performed at the level of populations, where there must be a net fitness benefit for positive selection. For your messy argument to work, there mu

Selection is performed at the level of populations, where there must be a net fitness benefit for positive selection. For your messy argument to work, there must be a change in the evolutionary context such that a trait that was once favored loses it fitness benefit, and instead any detrimental effects that were 'carried along' are now on display. There really is no 'hiding' in evolution; if a trait has deleterious effects, even only in specific genetic or environmental contexts, then they are felt. It is this brutally simple force that permits such complex systems to function effectively.