"So what happens if I want to charge my phone (e.g. from an external battery) and listen to headphones at the same time? Do I have to use some sort of ... dongle? That’s going to get silly."

What happens if you want to charge your new 12" macbook and you also need to use your cellular dongle ? They've shown total contempt for even this common, necessary use-case - why would they not show the same contempt for your use-case ?

"Admittedly, analog headphone jacks will go away at some point"

There is no reason to assume that. Only a childish contempt for "old things" and a desire to fix things that aren't broken would lead to that outcome.