The hard rectangular angles lead to fatigue failure from stress concentration. If more testing had been done and this had been known during the design phase, it

The hard rectangular angles lead to fatigue failure from stress concentration. If more testing had been done and this had been known during the design phase, it would have been possible to reinforce the rest of the airframe around the windows such that the airframe would be able to withstand the forces while retaining rectangular windows. No cracks, no crashes. However, the tradeoff for such strength would have been complexity, weight, and manufacturing difficulty; thus, modern airplanes have curved window ports to allow for windows while being light and simple. Aerospace and sports cars are the places in industry where lightness pays compounding dividends over raw strength. For a plane, if you can shave a pound off the design, you do it, since that's a pound you're not going to have to burn fuel to keep aloft for the entire lifetime of the aircraft. Or, for equivalent fuel, you could go faster instead. I suppose there's another question I never asked myself: What is the merit of a rectangular window? Why would we want rectangles on our airplanes? They're on our houses because they're simple to frame and create openers for, neither of which are important for planes.