Yes, there were obviously more factors that contributed to the Athletic's being worse.
But these are facts:

With Cespedes, the Athletics were 66-42 (.611). From the trade on, the Athletic's were 22-33 (.400).
With Cespedes, the Athletics led MLB in runs scored. In September and August, they were 22nd and 21st.


I think you are greatly de-valuing the importance of taking one of your best bats out of the heart of the lineup. It changes facets of the game that go deeper than looking at slash lines. I'm sure you've heard the term "lineup protection" before. It's very possible that losing Cespedes' protection contributed to the regression of players that batted around him.

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