I've got a few serious reservations about your theory.

For a start, you're relying too much on the premise that "one bad apple spoils the barrel". You can't judge this entire generation of rock music on one band - Greenday.

Then people get influenced by him or his band, are they better? No way. What I'm saying is that sooner or later music is gonna start to suck if people only keep on going down this ladder.
I don't agree entirely. There will always be musicians who build the foundations of music to come, with new styles and ideas, and they will influence other musicians to build on those foundations and improve the original ideas.

Concept albums came to the forefront of the music industry in 1967 when the Beatles released Sgt. Pepper's Lonely Hearts Club Band. There's little doubt that it's a fantastic album, but the only thing that makes it a concept album is the intro and the reprise. All the other songs on the album are completely unconnected with the idea of Sgt. Pepper and the Lonely Hearts Club Band.

Though there had been concept albums before Sgt. Pepper, it was the Beatles laid the foundations for this type of album. Sgt. Pepper isn't the best conceptual album ever made, but its ideas influenced much better ones in turn, such as the Dark Side of the Moon, and The Wall.