1. Mig-21 can't share cutting edge in BVR over even Su-30MKI. fixing a BVRAAM on pylon can't make any jet suitable for BVR Combat. You need BVR target and tracking.
If you cannot see, you cannot kill.
2. We prefer more value to our Pilots rather than any machine. Pilots are the most valuable asset for IAF. So, we can't put them on risk to fulfill "QUANTITY HAS ITS OWN QUALITY" criteria.
3. For you comparison , I have one more example which suits it most, 10 desi katta/Tamancha
can't outperform a M4 carbine or even an INSAS