The conversation has veered from tread directionality, and is beginning to lose contact with Vintage bikes...
May I offer a closing observation or two?
First, I don't remember seeing (many, if any) treadless tires Back Then. I wasn't in the Biz, and not often in the hotbeds of activity, but to me a vintage bike looks more normal with some tread. But, that's aesthetics, not performance.
Second, on the road (leaving out dirt, etc) there is a common sense argument about smooth v. tread, on bike v. car, that I haven't seen here: If you get down to the road level and look hard at the "texture" or average protrusion size on the tread and the road surface, whether concrete or asphalt, I think you'll agree that by far the bigger protrusions, holes, and channels are in the road itself, much larger than those on a bike tire. Bike tire typically has pattern with scale of 1 mm; car tire and road surface much larger. So, bike tire surface doesn't seem to have much dynamic opportunity to shed water except to the sides. Ergo, it doesn't seem that tread would matter on road surfaces.
But, wait! The most recent wipe-out I had was with a slick tire on wet leaves. I'm not sure that the same arguments apply in this situation.
We return you now to our regularly scheduled programming; that slick tire has moved from wheel to shelf, unlikely to see service again.
mcLean va usa