Also it’s not that the higher switching speed alone make for the good sound it gets, it’s also that the "low order output switching noise filter" that all Class-D’s have to use, can now also be taken up 2.5 x higher, this then has no effects now down into the audible range, like the filters today do, which can cause phase shifts down to 5khz, and why many complain about something not right in the upper mids/highs.
That's true, but it might open another bag of worms. In current designs filter leaves about 1% of switching noise on the speaker cable. It is only small percentage but of very high value, reaching 100V - making it approx. 1V peak switching noise on the cable. It can be easily seen with the scope. The only reason why it does not radiate is the fact that 500kHz wavelength is 600m. Antena becomes very ineffective below 1/10 of the wavelength - in this case 60m. There will be still some, very small electromagnetic radiation but it should not be a problem. Increasing switching frequency, let say 10x will make this antena "dangerous" at 6m, producing some radiation even with regular speaker cables. It is possible to filter it better, but it would lower bandwidth resulting in phase shift in audio band, that we want to avoid. Perhaps compromise is somewhere in-between, increasing switching frequency only 2-3x while still enjoying improved linearity thru reduction of dead time.