I may be thinking too hard, too stupid, or both. Should we not - if we were to mine the moon, deposit a mass equal to anything we retrieve lest at some point the lunar orbit be thown our of whack? Maybe leave Michael Moore up there, for example (if we can cobble up enough thrust to get him there).
Like the earth, the moon collects tons of new material as it moves through space. I doubt we'd ever be able to lift enough to offset that.