Bell Research in the past established a link between lower and upper frequency break-points wrt telephony clarity. As the lower frequency break-point decreased the upper break-point was increased (or vice-versa) to give a balanced spectrum for audio balance.
That 40-15kHz spectrum reminds me of that work.
My take is that while the 20-20 KHz range is easy enough to remember, it is more of an icon than a real world standard.
For example, most people who have downloaded and reported on using a CCIF 19+20 KHz file for listening, report that it is silent when they play it.
When CRT based TV sets had 15.75 KHz flyback transformers that spread sound at that frequency, very few but the youngest ever seemed to notice it.
A lot of perceptually coded files are found to be very satisfactory in critical blind tests and many of them are brick walled at 16 KHz.
Very few speaker systems reproduce 20 Hz audibly and cleanly, and life seems to go on for the vast majority of audiophiles without it.
FM stereo was low passed at around 15 KHz using passive analog filters that had wide transition bands and nobody seemed to complain.
Almost all common musical instruments have a note in their 32-45 Hz as their lowest ordinarily played note.
Since about 1978 an increasing percentage of LPs were cut with in-line low pass filters with relatively gently sloped and wide transition bands that intruded well below 18 KHz related to cutting lathe pitch automation, and nobody seemed to complain. In fact many seemed to say that they preferred LPs even though the vast majority cut up until about the mid-90s had this fault.
Conclusion - the 20 Hz - 20 KHz range is generally for almost all listeners overkill and far from being sacrosanct.