I'm a pretty keen Apple early adopter (first imac on release (pre-ordered), first iPod, first iPad (drove to US to get it), first iPhone (drove to US to get it)), but at the same time pretty much a regular user (don't upgrade to every version, usually skip several builds, don't "hack" anything, don't listen to music that much, etc.
I've been on board and praised the long term vision of Apple getting rid of legacy drives/connectors for a long time and seen how they were able (with intent I'd like to believe) transition the regular Joe consumer from a iMac with no floppy, to using CDs, to burning CDs to putting them on an iPod to downloading songs (and apps) from a online store, to Apple Music (for good or bad), etc.
There is no way they could be so strong in so many areas of the ecosystem from hardware to software while controlling the sandbox to keep things generally solid (3rd party apps, etc. ... (no more downloading something and finding it doesn't work with your version hardware/software or has bugs) for most things to "just work".
That being said, I think getting rid of the 3.5mm jack is functionally big mistake, but will be minor over time.
Aside from the technical (waterproof or thickness tradeoffs I can't really speak to, but don't think the previous phones or competitors phones were doing too bad), the move breaks the main function for a lot of users - the ability to listen, (and while while charging), on any headphones, anytime. Headphones are ones of the main ways people personalize a device from what I've seen. Some want audiophile quality, some want cheap, etc. .... some want to pretend they are an audiophile, but don't know anything and buy Beats.
I don't listen to much music at all, but when I do I use the $99 in ear Apple earbuds (normal ones fall out and I hate big cans). I use them on a plane, while going to bed, and the odd time just walking around. I'm pretty careful but still they seem to break at one end or the other about once a year or so, and I replace them. I also always travel with a backup set of the cheap ones that came with the phone, just incase they break mid flight on a 15 hours flight to China...
Will the new Apple ones fit in my ears? Will enough 3rd party manufacturers get on board to offer alternatives? Will all earphones be bluetooth in the future and concerns be moot about battery life and sound quality? Will the charging issues go away or not be a problem for majority of users (complaints about them not lasting a long flight are valid, but no device has a battery that lasts forever, laptop, phone, etc.)
I don't know the future, but I can learn from the past.
In the meantime, I'm not thinking it's a cash grab for dongles, but a mid term solution and will work it itself out like other "big problems for Apple" such as -
-no floppy drives
-no ethernet ports
-no replaceable batteries
-no memory sticks on mobile devices
-no usb ports on iPad
-no stylus for iPad (the Pen thing is a special case, most are fine with fingers)
Indeed there are valid concerns about losing earbuds, dealing with stupid dongles, etc. but if it allows Apple to innovate ahead and they saw the 3.5mm jack as a barrier to doing some things in the future (brain implant wireless control, 3D sound or multiple device input, etc.), maybe it is good to break with the past and move ahead. I can't think they would eliminate the 3.5mm jack just for fun to piss people off or to sell more Apple AirPod Beats...
PS. Happy that I'm typing this on my Bluetooth wireless keyboard, not an old ADB one with curly telephone cable cord