But yeah, the development of tonality kinda stopped in the 1850 range, with maybe Stravinsky, Tatum, and the 1940s Bebopers extending it as an exception. But at that point, tonality had passed out of the "art world" -- everything post-Duchamp's "Fountain" has been a bit more deconstructive and nihilistic.
Maybe of note, pun intended, Schoenberg was pursuing the "emancipation of the dissonances" while the Bolsheviks were destroying Russia. And of course, that's part of why nobody listens to Schoenberg or performs his stuff -- it's grating and kinda horrible, at least after he abandoned high Romanticism after Opus 5.
To be clear, I agree with you on the mindless labeling of stuff that's in current music pedagogy at universities -- that's mainly out of Yale. With my students, we wrote Lieder.
Maybe -- but the context to understand what Debussy and Ravel were doing (maybe not Scriabin) is to get the Bach / Haydn / Mozart / Beethoven / Schubert under your belt.
Plagality is a pretty deep musical concept within tonality, though -- you have to deeply grok counterpoint and tonal harmony first, and then you can start to hear and perceive different aspects of tonal gravity.
For plagality, it's the "dark side" of the tritone, specifically with iiø or iv6.
Start putting together that tritone with the major standards of vii° or V7, and you're really rolling with the joy of traditional harmony.
Yeah, Rameau is the "Louie, Louie" French fellow who was the "Newton of Music" who gave us the Roman numeral notation, while the Germans were still doing counterpoint and harmonics.
I very seriously believe there's a link between that, the revolutionary attitudes and kinship of the French and Americans, with the Rock and Roll movement of the 1950s / 1960s.
Coming from this field personally, this feels like typical Temperley and de Clerq sorts of things. Davie Temperley was in David Huron's lab once upon a time, I believe.
The systematic musicology world, especially the portion doing corpus studies, often is just doing descriptive research. The article linked is a bit more prescriptive.
The best book on music theory ever written, IMO, is David Huron's "Voice Leading -- The Science Behind a Musical Art." Definitely recommend.
There's a general principle that's at work here for certain concepts: it's often a good idea to teach children technology skills in terms of the technology's historical development.
Riding a bicycle is certainly one; the earliest velocipedes had no pedals at all, and are essentially the "balance bikes" mentioned here.
Mathematics and computer science is another one -- no matter how elegant it can feel to construct a world from teaching in first principles first, the historical pedagogy is much more intuitive and effective, IMO.