The Soviet Union recognised that the MiG-21's time as a frontline fighter was long over when they began the Bis upgrades (and before then).
Mission designers might think about the idea that Warsaw Pact doctrine was not to operate MiG-21MF and Bis as air superiority fighters, they were assigned to support units flying lightning strike missions against tactical targets following a comprehensive recon-in-force mission from aircraft like Su-24s.
The idea behind Soviet armour deployment of the 1970's/ 1980's was to use fast aircraft in large numbers to eliminate soft targets and potential ambushes before rolling very large numbers of tanks and IFVs into the theatre to overwhelm enemy forces on the ground.
This was the initial logic behind the development and employment of the MiG-21Bis in Soviet Union service, as well as being a natural choice for a relatively capable, cheap upgrade of an already popular aicraft to sell to Eastern European satellite states and third world allies.