While I am really familiar with that film and the players in it, I think your point is rather limited to considering "pop" music, which creates the biggest returns. None of the 'hired studio musicians / songwriters / etc' dynamic applies to a band like Muse, Coldplay or Radiohead, yet still applies to Adele and Taylor Swift. The Wrecking Crew film is a very good summary of the way 'the business' worked long ago - still does to a certain extent, say in Nashville - but the access to instruments and music now has really changed the dynamic.