what is pleasing to the human ear has not changed since man began writing music

is true.

No, it's not. I understand your point about the diatonic scale. So yeah, it's true that off-pitch notes that aren't in the proper scale will sound bad to our ear. Or that even certain legal intervals within that scale will sound dissonant.

But you said it yourself: even-tempered scale. Before we settled on the even-tempered scale, we were tuning the instruments to scales that weren't even-tempered. Some of the classical music we listen to today was composed for non-even-tempered scales, and would sound slightly out-of-tune to our ears if played that way today. (You can even find modern recordings of such performances just for reference.)

And then you have to think of how someone from the 14th century would react to a rap song or a distorted guitar. Or heck, even a piece of improvisational jazz.

I agree completely with your point about songs needing to be in tune and not too dissonant to be hits. But does a record company really need a computer program to tell the A&R people whether or not a song is in tune? If that's true, the company needs to fire its A&R people.