Aside from a puck mouse, and a trash can Mac Pro, Apple has historically preferred squares and rectangles. Though the original iMac was curvy, from the face it was still generally square shaped.
most screens are generally "square shape", its called a rectangle (2 sides longer than the other two but of equal length value) and in the past ALL screens were square unless you go back to the TV's when they were first introduced. Your post, while I agree that apple watch will remain "square shape" for some time, was rather short minded. A circle apple watch could easily be done while maintaining the same band attachment. Picture a normal mid to high scale round watch, they have basically what the click and many other adaptors. I think that even if they release a round AW (I don't think they will) the bands will still fit. I make a guesstimate that the current bands will be used in the AW for at least 2 more generations which seems that to would be 3-4 years.