Ah, OK. It makes sense that pretty much whatever's already out there is all that'll ever be out there, so I take your point, that each one that lands somewhere else is no longer a threat, so that in time, presuming that they do land somewhere else as time goes on, our risk drops, all other things being equal.
But consider: The ones that do land somewhere else were never threats to us anyway, and so it must also be true that those objects that are going to land somewhere else, are also not currently threats to us, whether or not we've detected, characterized, and ruled them out yet. So in a way, our risk remains the same as time goes on, since there is either a 'big one' out there already, heading our way as we all live and breathe, or there is not. If there is not, then our risk is right here right now zero, regardless of whether we know that or not, and if there is, then we aren't 'at risk' at all, since by definition it's a 100% certainty that we will be struck.
Given that preparing for an impact event will teach us certain things that we would not otherwise learn, as for example the Apollo program taught us things we would not have otherwise learned, I cannot believe that this preparation would literally be a complete waste, even if we are really at zero risk of ever being struck. And obviously if we ever are struck, then preparing for it is completely justified.