It would be very un-Apple-like to backtrack on USB (much as I'd like them to), but reintroducing HDMI and SD wouldn't be a backtrack.
The difference is that one could reasonably assert that the industry will eventually migrate away from USB-A (even if the timeframe for that is highly debatable). But there is no case at all to be made for moving away from HDMI or SD – the idea of using USB-C ports instead of HDMI on TVs and projectors is only in its earliest proposals, and there is nothing on the camera industry's horizon to replace SD cards with any kind of USB-C compatible media.
USB-A may be superseded (if only technically) by USB-C, but HDMI and SD have no current successor.
Cable length is an issue. A fifty feet HDMI cable at Monoprice is less than 25 USD. A similar USB 3.0 cable needs to be active and costs 100 USD. My mind boggles at the thought of a 50ft USB C cable.
The only reason HDMI still exists over the vastly superior DisplayPort is the cable length. The DP standard, if memory serves, limits the cable to 3 metres / 10 feet so the max cheap passive cable you can buy on Amazon is obviously 15 feet. Meanwhile, the HDMI standard has no cable length limit but in practice 50 ft is the longest passive you can buy.
This discussion was about replacing HDMI with USB C -- I presumed on both ends. Switching USB C to HDMI alt mode and then running a HDMI cable is not replacing HDMI with USB C. Let's not even mention I am unaware of any product switching USB C to HDMI alt mode. All the available USB C to HDMI converters are using DisplayPort alt mode and an active converter, packed in one IC.
10gbps, 60W USB C-USB C cables are not even available at more than 6ft. It's too complicated a standard for that. You'd need fiber and USB-fiber extenders steadfastly refuse to come down in price even at 5gbps and I haven't even seen a 10/20gbps.
The difference is that one could reasonably assert that the industry will eventually migrate away from USB-A (even if the timeframe for that is highly debatable). But there is no case at all to be made for moving away from HDMI or SD – the idea of using USB-C ports instead of HDMI on TVs and projectors is only in its earliest proposals, and there is nothing on the camera industry's horizon to replace SD cards with any kind of USB-C compatible media.
USB-A may be superseded (if only technically) by USB-C, but HDMI and SD have no current successor.