Soviets themselves never flew a single Mig21 beyond 25 years.
India Approached Russia after 1989 saying is it fine to operate Mig21 for a longer period of time if we upgrade it?
Soviets took the money and gave a piece of paper saying yes we can.
And ever since we are suffering.
When Lockheed Martin says fly the F16 for 40 years, we see USAF flying the F16s today (having 8000+ hours clocked).
We made a mistake. Mig21 was never engineered to be operated beyond 25 years of life.
Even the Chinese copy F7, you can yourself check, Pakistan has already started retiring F7PG which are (surprise) reaching 25 years of age.