And let me be clear - the 3.5mm jack is far from being the perfect way of transmitting and sending audio. It's a vestige from an analog world where you have to take impedence and resistance of the load into account, and I don't use it much myself.
But the use-case where you have to stream music from your phone and you have just a humble set of speakers, an amplifier or some old headphones isn't that uncommon - especially for musicians like me: what happens when you're in a studio with your band, you want to play an mp3 on your phone, but you have no way of plugging your phone to the mixer or the amp?
Pretending that such use-cases don't exist, and force all the users to figure out an alternative, is an act of sheer arrogance on the producers' side. And such arrogance isn't even justified by the claim of water resistance: it's technically possible today to have an audio jack port AND be waterproof.