Dan, I think if you used an async USB input that you would find that USB is very, very competitive with SPDIF. Some would argue better.
The Ayre QB-9 is a fine example of a superior USB DAC which is fairly robust against quality of computer transport.
As you no doubt already know, Ayre licenses their USB technology from Wavelength, while Empirical Audio and Wyred4Sound licenses theirs from M2Tech.
I am not personally looking for a new DAC right now, but if I were shopping and you included an inferior USB implementation, I'd need to consider the price of your DAC and factor in the additional cost of a very good indeed USB to SPDIF converter. I think the market is increasingly moving to computer based transports, and async USB is the bet I'd likely make. And it turns out the bet that Wavelength, Ayre, W4S, Empirical Audio are already making.
Having said that, my present DAC is not an async USB DAC, but I needed to buy a new 2010 Mac Mini to make that work for me. Not sure if most of the market would want to do the same.