[Bakary]

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.

[mtlmtlmtlmtl]

Right, that makes sense. I honestly wasn't aware of the Back to the Future inspiration, I watched it as a kid and it was never really my type of movie.