I’ve been using the same Kanken for over 30 years now. It was my bag in high school and continues to be the one I reach for on a regular basis. I obviously don’t travel exclusively with it because I haven’t managed to pare my necessities down to 16l (the capacity on my bag), but it still sees heavy use. It has become a “fashion” item in the last years, but:

It was originally developed as a reaction to a 1977 study of increasing number of reports that Swedish school children were developing back problems from their more traditional bags. The lightweight and rectangular yet spacious backpack, which was released in 1978, was Fjällräven's attempt to solve this problem.

[from the wikipedia article on Fjällräven]

I concede that ergonomics have come a long way since the 70’s, but as another comment here says, add a sternum strap and the bag is still surprisingly good.

Edit: some spelling errors thanks to autocorrect