That's not really the issue though, you have two devices that you have to plug-in, the midi keyboard and now the adapter which means you have to use a splitter or usb hub. In my experience it's not usually very seamless and can sometimes cause issues with detection of the midi keyboard properly.
EDIT: I have no idea where you're getting that figure but I just looked at three different pairs of Bluetooth headphones that I have in my house, Soundpeats, TOZO, and an LG. None of them have 3.5 mm jacks on them.
Unfortunately your average consumer is not going to miss the inclusion of a 3.5 mm audio jack, but it really sucks if you're a musician.
There are a bunch of really good soft synthesizer apps for iOS that work with a midi keyboard but Bluetooth latency makes them virtually unplayable. With a standard 3.5 mm jack it's not an issue. You can buy a dongle but that's just one more piece of complexity.