The point is, that before there were drug tests, there were employees that used drugs and no one ever knew about it. It seems that if drug testing was dropped, then, as you said, there would be more employees available. It shouldn’t be a reason for not hiring someone. The lower-level managers and supervisors would actually have to judge people on their performance and attendance.
A key point here is that we have expensive, dangerous machinery which could kill someone instantly if they aren't paying attention.
Yet, we've had no injury more serious than a severely sliced finger in roughly two decades of 24/7 production at our facility. Who can make a coherent argument that drug tests are unnecessary with that type of track record?