Kops, you can go to AES/EBU, S/PDIF or TOSLINK to achieve 192 kHz/24-bit operation with the Playback Designs today.
I'm not sure what you mean when you say "future proof". With USB, the PD's firmware can be updated (it has been a couple of times since I've owned mine) and there are ample ways to use external devices to obtain 192/24 operation in the current hardware/firmware configuration. No device is likely to cover all potential future hardware possibilities, since we won't know them until we see them.
Dave