To me, having USB out in the new Quartet M-Scaler would just open a new can of worms.
If people try other DAC brands with the Quartet, and they say they don’t like it, it could be:
1) Their DACs could not adequately resolve the signal optimally (or can’t take full 768kHz output)
2) Their DAC USB input has a poor implementation
3) They just don’t like the WTA filter + noise shaper
But instead of attributing the problem to themselves or their DACs, they will naturally blame the Quartet because they are not going to blame the gear that they already own and love. And maybe the problem could be coming from Quartet but who knows.
I guess in theory, having USB out could potentially increase sales. But it could just increase the amount of online criticisms on the product that could have nothing to do with the product itself. And of course, adding a USB out would also delay other product development down the pipeline. So to me, adding a USB out has a lot of potential downsides and minimal upside.