What complexity to the interface would I as a user see with a microSD card used for media?Problem with that idea is that fast storage is necessary for the OS so things load quickly--many apps are up over 100MB now. SDcards are good for secondary storage, but once you go there, it adds complexity to the interface and the OS. Google knows this, so Pixel (and Nexus before it) also lacks a microSD slot. Android has been toying with moving away from this storage type since 5.0, because this storage medium can be easily removed and accessed outside of the device, posing security issues. Onboard storage, not so much. Also consider that MicroSD is not a very reliable storage medium. Of the 5-6 cards I've owned, about half of them have gone bad and failed to read anymore. It's not as simple as adding the slot anymore.
64 GB internal storage is more than anyone needs at this present time for OS and apps.
Samsung learned to their cost not to remove SD card storage.
And don't skimp on SD card quality.