No, the original sentence is absolutely correct. Only if eating more spinach quit being beneficial would we use the linking verb was .
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fivejedjonBoth forms are correct.There is no need to backshift, but backshifting is rarely incorrect.I wonder why someone 'disliked' that. If they think I am wrong, perhaps they could suggest why.
VorparCynthia explained why it is less ambiguous to use present tense.She didn't explain that. She said, "... the original sentence is absolutely correct". I agree.