If that's the case, there's no point in doing anything at all!
Monitoring interlopers from extrasolar space (the ones that miss us, anyway)is certainly worthwhile for scientific reasons, but not for planetary defense purposes.
Also, its a whole different problem keeping an eye on solar system objects, which tend to stay close to the ecliptic and are all traveling in more or less the same direction at predictable speeds, than extrasolar objects which can come from any direction and have a much wider velocity distribution.,