There's already an instruction manual for writing pop hits: The KLF's The Manual (How to Have a Number One the Easy Way) is a (tongue-in-cheek) step by step guide to achieving a No.1 single with no money or musical skills.

https://en.wikipedia.org/wiki/The_Manual

Sci-fi story of note: "The Ultimate Melody" by Arthur C. Clarke[0], from 1957.

[0] https://archive.org/stream/1957-02_IF#page/n71/mode/2up

David Cope (a professor at the college I went to) started working on that idea back in the early 80s, calling it "Musical Intelligence". His approach got the most attention for its ability to write new classical works that accurately mimicked the style of famous composers.

http://artsites.ucsc.edu/faculty/cope/experiments.htm

Never, because "perfect" in terms of entertainment, "wow" factor, and endearment is a moving target and a product of the times. If things get overplayed, they get stale.

So we constantly get new songs that reuse the same familiarity in different ways, but not literally the same replayed song.

No such thing. But AI will definitely write pleasing music in the next 10 years~.

i think we may also see the "perfect" ad jingle or notification tone, this could also be used for consumer conditioning

for examle; playing a likeable sweet sound when consumer does something good [clicks on ad] or a sour tone when doing "bad" like blocking an ad or popup.