It sounds like the experiment, as designed, reached a conclusion in far less time than 69 years. It just took 69 years for someone to think about video taping it.
Maybe it was a badly planned experiment (or more likely, the cost/availability of cameras in those days prohibited their inclusion in the plans). Nevertheless, no firm conclusion can be drawn that the experiment could have been ended early, those previous "drips" may not have happened without external forces applied and so even if cameras had been used it may not have ended until now. Its probably likely that it would have ended had cameras been available, but the scientific method generally calls for a greater degree of certainty than "probably likely" before calling an experiment a success.
And of course, it is called an "experiment", not a "certainty". Mistakes happen and they're not always optimal, that’s all part of the fun of science!