One fairly decent argument against waiting until we have the technology is that it cuts against how we develop the necessary technology in the first place. We can't improve our propulsion/spacefaring tech without designing, building and launching spacecraft. There's absolutely no reason to assume that we will have invented a faster engine if we've just been sitting around for 100 years not building anything. Hell, we're having trouble rebuilding a moon program fifty years after Apollo 11, inspite of all the advances that have happened in the meantime.