Airframes, engines and aircraft performance are just one small aspect of an overall combat system. Modern air combat has gone way beyond these parameters. Compared to the Pakistani older version of F-16s, Mig-21 Bisons have vastly superior radar, BVR weapon suit and other on board computerized and electronic warfare systems. The difference is between having them and not having them. A modern combat aircraft isn't simply about engine and airframe. Pakistanni F-16s block A/B versions will not even stand a chance to come close up for a fight where it will be able to used its superior agiliy and airframe advantage.
And I am not even considering the combination of AWACS and Mig 21s against F-16s. In which case even the modern F-16s block 50/52s would be easy targets.
Even otherwise Mig-21 has greater speed then an F-16. A skilled pilot would know how to use it to its advantage. It the reason why USAF F-15 pilots (by their own admission) found it difficult to lock on the the Mig 21s at the Cope India exercise.
Why else do you think Pakistan (with China’s help) still want to buy/built modern day clones of modified Mig 21s even when it has (or can buy) F-16s?