Saab pointed out during the test programme that variable intakes only make a major difference in performance above M1.4. Seeing as how little time the aircraft was expected to be in that regime it was decided the weight and complexity wasn’t worth the expense and time. Subsequent history would seem to have proven their case.
It should be pointed out the fixed Lightning bullet also served perfectly adequately during the same era.
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https://eprints.soton.ac.uk/46202/1/AIAA-26830-529.pdf