About the song becoming a mess: That's why I specifically mentioned classical music, where the whole point is to introduce a motif and to modify it in various ways. There are countless symphonies that span anywhere from a dozen minutes to almost an hour with incredibly rich thematic material, complex and intricately composed masterpieces where every note for every single instrument has a place and a purpose. Let's not underestimate listeners. People can listen to stuff like that, analyze it and follow it perfectly well. To claim that long songs need to repeat the same section exactly as is, is almost patronizing. Fugues for example are entirely based around manipulating one single idea and creating a piece out of that. Furthermore, you can callback without repeating the same motif as if copy/pasted. You can modulate, you can change the chords, you can add harmonies; there are a million ways to make it more interesting than just repeating what came before.
