Mig-21 in service with VVS in late 1980s

I’m a big geek when it comes to comparing inventories of airforces and I was going through the inventory of VVS in the laste 1980s and it seems like that there were atleast 500 Mig-21s still in service with VVS in the late 1988 even though 1000+ mig-23 and almost 400 mig-29 were already in operational units
What could be he possible benefit of retaining so many mig-21s so late in the cold war were much better planes were plentiful ?