I don't think they ran out of sci-fi tropes so much as they ran out on the core idea of the show. They took two established characters (Marty and Doc Brown) and

I don't think they ran out of sci-fi tropes so much as they ran out on the core idea of the show. They took two established characters (Marty and Doc Brown) and explored/deconstructed the inherent absurdity and great dynamic between those two that was never fully exploited by the original films. The decline started once they had done what they could with it.