He was so good at ending the episodes on a that multi-dimensional high note- it was a hard thing to keep doing. At least to his standards.

I think it was more of not wanting to disappoint himself/others in that role than any expectation that the show would go downhill or was on the verge of getting stale.

But that's just my layman's take.

There's plenty of early era blog posts written on the topic by people more informed than me, though. As well as dead-tree entertainment magazine articles.