Cost <$100 to "isolate" USB. Most tolerable DACs already isolate SPDIF (something that most audiophiles are clueless about -- going on about isolation, TOSLINK, etc.). So there goes your isolation argument. Isolate USB and with SPDIF on most tolerable DACs, no RF, there goes that argument. Timing? No timing in USB, not timing in Ethernet (also isolated and fairly immune to RF too). $15 DAC chips can remove nanoseconds of jitter, so that there goes that argument.
I don't want to misinterpret you or misquote you, so could you please write this more clearly before i reply? For example, are you saying that timing/jitter does not matter on the USB interface? If so, you are confusing a purely data signal with the quasi-analog signal that is fed to a DAC.So, please clarify the whole thing. Thanks.
G