I mean, USB-C seems to be the direction we're going anyways, plus it offers a lot more flexibility for audio eg. allows for active components like a higher-end DAC or amp on the headphones.
Most people probably wouldn't really care for this, and the ones who do probably have portable media players already, so I would prefer to keep the headphone jack. The reality is, though, that USB-C appears to be the future.
You don't need type-C for running an external DAC/amp. Most smartphones support USB-OTG over micro-B and many DAC units use generic audio drivers.
Of course you'd still need two ports if you wanted to charge at the same time.
It would probably be more interesting to see smartphones that could use the headphone output as a digital signal, like a coaxial output. We want the 3.5mm output anyway, so why not just let it dump a digital signal to an external amp?
I prefer keeping the headphone jack, but am not against a second USB-C port. If you make the device thick enough for a headphone jack to work properly then you've got room for a reasonably large battery and the extra ports without any issues.
Mobile phones are thermally constrained these days anyway.
Most people probably wouldn't really care for this, and the ones who do probably have portable media players already, so I would prefer to keep the headphone jack. The reality is, though, that USB-C appears to be the future.