With the other types in IAF, these surely serve as last line of defense, it still has excellent climb rate. Most IAF MiG-21 crashes were reported to be pilot error because of lack of training, birdstrikes, or overhaul/maintenance inadequacies as far as I know. There's nothing wrong with the type, it's just old and not the easiest to fly. It is a money and maintenance thing, as well as requirement, if India would have retired the type, it would have a capability gap right now. Those M-MRCA are not there yet, and the LCA has suffered from lots of delays.
I think the BISON upgrade also gives them a one piece windshield for a better view.
Yes, they did wait long, but don't forget that the upgrade is finished now, not just started. I think most India's MiG-21s which remain in service are not as old as you might think, licensed production ended in the mid 1980s.