If you try the experiment lots of times with drops of different sizes you find the oil layer always has roughly the same thickness. That's an interesting observ

If you try the experiment lots of times with drops of different sizes you find the oil layer always has roughly the same thickness. That's an interesting observation that calls for some kind of explanation, and the hypothesisis that the thickness of the oil layer is the length of one molecule is perhaps the most obvious and plausible explanation. Then one would look for confirmation, of course. (What was the next thing to confirm this, historically?)