I am not Tim but sure .Tim,
It is something that puzzles me. The performance of analog semiconductors has improved by several orders of magnitude during the last 25 years. Computational power of processors also increased by several orders of magnitude. ADC and DAC improvements were huge. And we still say "it is extremely difficult to design system performance at frequencies greater than 20 KHz. How can we explain it?
The difficulty is not due to tools and electronics but my ears! I can play a 10 Khz tone and hear it. If I play a 30 Khz tone, I can't. What would be a better response? One that rolls off the high frequencies at 30Khz vs 40 Khz? What if I have less distortion at 30 Khz but more noise and reverse is true for 40 Khz and vice versa. How easy is it to test that scenario?
Take what Lee mentioned earlier. We know from specs of DACs and how they work that distortion and noise increase when we sample at higher frequency. How do we know that is the better trade off for sure?
We lose one of our best instruments at these hpyersonic frequencies. Our ear.