Veteran producer Rick Beato and a guest discuss the question, “has every song been written?”, in the video below. Short answer: no, but the harmonic palette, in particular, of popular music has in recent decades been blanded down to such a degree that it becomes ever harder to do something unique within it.
Economic constraints — particularly the ever-shrinking revenue pool of the music industry — also make A&R people of record companies and songwriters-for-hire ever more risk-averse . The end result is what is known by a priceless Dutch word as “eenheidsworst” — standardized sausage.
Spin, strangeness, and charm 
I believe Brandon Flowers of The Killers had this exact question.
He answered it in a song of the same name.