All John did was have written three novels.
Is this sentence grammatical and idiomatic?
I think what's happening is that only infinitives (and in this case to is not specified) are valid after to be.
If you want to express state, then you'd need something like "John's entire (creative output / life's work) was 3 novels". If you explicitly use a verb, you're no longer expressing state: "The only thing John ever did was write 3 novels".
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