What we lose if the ISO designates the 'wrong' day as the first is continuity with ancient documents and those languages which still know days by number rather than name, although it sounds as if there is an existing problem with East Africa even then.
It depends whether you think continuity matters. The ISO apparently doesn't.
When i was a a kid, i remember having a discussion with other students about this.
Some were saying the first day of the week was sunday and i,among other students were saying it must be Saturday because Sunday being the first day did not make sense.The argument was because "Sunday" is "jumapili",as in "day two" and it did not make sense to call "day two" as the first day.Apparently,at the time,everybody in the world spoke swahili and what does not agree with the language was not correct.Being a kid and not knowing a bit more is fun sometimes.
"Wednesday" in swahili is "jumatano", "tano" means "five" so "jumatano" translates to "day five". Making "Wednesday" the first day of the week may not be very problematic in english but calling "day five" as the first day of the week may seem odd even to a 2 year old.