Each has its own role to play. It's a Hi-Mid-Lo mix. Currently F-16s make up the Hi-Tier, JF-17 the Mid-Tier and the legacy fighters make up the Lo-Tier.
In the future Pakistan may opt to make up the Hi-Tier Twin-Engined Flankers and eventually the 5th-Gen fighter. The F-16 and the JFT-Block-III would make up the Mid-Tier and the rest of the JFT would make up the numbers with a decent punch.
The Hi-Tier fighters cannot be too many in numbers due to their high cost of acquisition and operation. That is why a potent Mid-Tier and decent Lo-Tier is necessary to make up the fleet numbers and collective strength.
Then, also factor in the scenario, sometime between 2023 to 25, we will be facing 300+ AESA upgraded Su-30MKIs, Rafale and maybe even FGFA. In such a scenario a handful of Hi-Tier and some AESA JFT would find it an uphil task to ward off this threat. Our Vipers would be hard pressed to fulfill its missions and role with the current PD Radar.
So if we get an AESA upgrade for our Viper fleet then OK. Else, we may be forced to induct J-10B/C to make up the potent Mid-Tier.