The design fail is not taking the obvious and consistent (albeit spaced out) need to charge the mouse into account. It's not like some people don't have to charge this mouse; everyone will charge the mouse at some point, and they will do so consistently over the years.
With this design, I get a first class experience using it when the batteries aren't empty, but I literally cannot use the mouse when I need to charge it. The idea that a mouse should sacrifice such a basic functionality requirement just for the sake of a slightly better design is hilariously backward.