Hmmm. An interesting thought.
If you want a game series that supports your thesis, the Mega Man series would be an excellent choice. The classic and X series have some of the best music in the music world, but the quality has slowly declined. The Zero and BN series had good music, but not great, and Star Force, Legends and ZX series have generally forgettable music.
Now, this does not mean that every track in those series is exactly like that. There are some classic and X songs that aren't very good, and there are some ZX tunes at least that are pretty sweet (look for Green Grass Gradation, one of my favorite tunes).
And I don't know if I agree that it has been declining in quality, mostly because composers nowadays have FAR more tools at their disposal. Also, a lot of music tracks nowadays are very ambiguous and ambient (listen to all of Portal 2's soundtrack. It's really very minimal) due to the lessened limitations on their tools.
Also, there are some GREAT indie game composers out there, like Danny B of Team Meat, who still make dang good music. You just have to look around a bit.
Good luck with your paper!