Brahms's attitude wasn't just an eruption of artistic temper but signals an essential sense of history. It's been said that there are only a half-dozen elemental themes in literature and that all the great books, poems and plays ever written are mere variations. So with Western music, which is based upon a handful of common assumptions. Melodies and chords coalesce around certain intervals, harmonies evolve, rhythms develop and structures progress in certain ways because we find them pleasing or expressive. If that's true then, at least according to the thinking of the Chiffons court, whoever first thought of those three chords is owed billions in royalties.
Musical copyrights are fundamentally unfair.
Novelty is at best a matter of degree. Many of the early copyrights were simply a matter of having been in the right place at the right time. And in this case the right place was a studio and the right time was the dawn of radio and the recording industry. Most such songs were largely derived from the musical traditions in which the artist had been raised. Without actual evidence of a previous version, challenges were rare.