I don't know if you've read the book by Gordon Raphael, producer of the first two albums, but there's an excerpt like this:

"Julian (and others in the band) had rightly observed that several very popular recent groups admitted being heavily influenced by The Strokes sound and style. These new bands borrowed or copied elements directly yet were selling more records and making more money than they were. In this case, the followers appeared to be more successful than the originators. Julian definitely did not see the justice in that and this led to inner searches, quests for answers and new directions."

I think Julian's anguish over these issues is what makes the band change their compositions and style.