I may have raised this point elsewhere in this thread… but I don’t know if I would agree that the Beatles deviated from “traditional” progressions and melodies. I think a historical footnote worth keeping in mind here is that in their earlier gigs the Beatles were playing a mix of original compositions and pop-music-of-the-day, which today are songs we would recognize as jazz standards. And, accordingly, because they were pretty well versed in that cannon, they themselves tended to write a lot more non-diatonically than we might associate with pop music today, but in ways that would have sounded pretty familiar to pop music listeners in the mid-60s.
Nirvana… again, I don’t know if I would agree fully, either, for different reasons. Kurt loved the Beatles and clearly got a lot of his melodic sensibilties at least in part from them, but he also drew pretty heavily from punk, which WAS more diatonic. Running through Nevermind, Teen Spirit was entirely diatonic, In Bloom was diatonic, Come as Uou Are has some chromatic passing tones but the “bones” of the riff are diatonic, Breed was diatonic, Lithium is maybe the first time you hear something non-diatonic, with the inclusion of the C in what’s otherwise a E (down a step, so sounding Bb in D) major song, and that sort of major-to-minor-for-tension move is as prevalent in folk as it is anywhere else - it’s not like Kurt was writing flat-5 substitutions here or using voicings much more complex than power chords or standard bar chords.
Either way - I thing the important takeaway here is writing with harmonic movement that extends outside vanilla diatonic harmony 1) can sound “normal” and not call attention to itself for being especially “jazzy” and 2) is worth experimenting with because given how heavily into diatonic harmony pop music has gotten in recent years, even just dropping in a half-step-up key change for effect at some point can make a song “pop” in the jumping-out and not genre sense.
Thinking out loud, the internet has been an awesome resource for learning music theory. But, I also wonder if it’s maybe been a little bit of a curse as well as a blessing, as it’s never been easier to learn the underpinnings of diatonic harmony and then when you do, think, “great, this is the set of rules I need to make chord changes sound good,” and it may in part, paradoxically, be responsible for at least some of the dumbing down in harmonic complexity we’ve seen in pop music, augmented by the last real wave of new music before the rise of the internet was heavily influenced by punk bands, which took dumbing down of harmonic complexity as a badge of pride and a bit of an assault upon the musical establishment.
