If you read the relevant papers or my book, you will see that the "Harman" curve is a steady-state room curve, including reflected energy, measured at the listening location. It is what is measured from well-designed loudspeakers that received very high sound quality ratings in double-blind listening tests. These all have a flat on-axis (direct sound) response, and well-behaved off-axis performance. Because lower frequencies - longer wavelengths - radiate more widely than high frequencies, room curves will rise as frequency drops - so there is a downward tilt in the room curve. Perceived sound quality is strongly influenced by direct sound, which is radiated as a "flat" curve. When measured at the listening position there will be an additional small very-high-frequency downward tilt because of air absorption, which is related to listening distance, something that human listeners appear to take into account, as it applies to live sounds as well as reproduced sounds. It is not, as you imply, a "Harman" decision to attenuate the high frequencies.
A person, who by virtue of age, noise exposure, ototoxic drugs, etc. has lost the ability to hear the highest frequencies will find little or no benefit from a high-frequency boost. The loss is usually sensory-neural - the "microphone" is broken - those frequencies are gone.
And, finally, I will repeat that the "Harman/Toole - you name it" room curve is not a generalizable "target" curve. It is what you are likely to get from a well designed loudspeaker. Adjusting a less-well-designed loudspeaker to have the same shape of room curve will not produce the same sound quality. There is no substitute for starting with a properly-designed loudspeaker.