That flaw in the "weekend" argument you point out is actually where I realized Monday–Sunday that I grew up with wasn't as obvious as I thought. I like Sunday–Saturday mainly for the structural symmetry. (This is also somewhat cultural, but I think most places nowadays would standardize around Sunday/Saturday being stereotypical "off" days.) Every week starts with one stereotypical "off" day and ends with one stereotypical "off" day with five "business" days sandwiched between (thus "Hump Day" too is the exact middle of the week rather than just the business week). It's not that big of a deal, but I think it's cleaner. Unlike 24-hour time versus 12-hour, I don't have a solid empirical argument. I'm wrong by ISO standards, but then then MDY and DMY are colloquially used much more common in most places than YMD, so I'm rarely abiding by ISO standards there.