Didn't quite a few staff members leave after just about every game was published, starting with Goldeneye and culminating with the Stampers hanging it up? The Conker project was long-in-the-works, as was DP/SFA, so you could maybe assume that those teams were constituted during the height of their creative prowess. But then you've got Mickey's Speedway USA, DK64, Jet Force Gemini (people have a lot of nostalgia for this game, but I don't think it stands up next to BK or PD), and the announced Kameo (which looked lame to begin with and turned out to fulfill that promise). Then Microsoft era: Grabbed by the Ghoulies, Conker port, Perfect Dark Zero (hiss, boo), BK: Nuts and Bolts, Viva Pinata (which was actually pretty good), and finally Kinect Sports.
I think Rare looked on the wane toward the end of the 64, and never really recovered creatively. The question might be how much of the staff defection and creative decline can be directly attributed to Nintendo's meddling. Probably a lot. I used to read the Uncle Tusk and Letters columns back in the day, and in the last few years they were letting quite a few thinly veiled jabs fly at "Ninty", culminating with that Mr. Pants Christmas Tree image with what were clearly a PS2 and an Xbox wrapped beneath it.