Complex sentence: The principal and most obvious goal was to replace India's ageing MiG-21 fighters, which had been the mainstay of the IAF since the 1970s.
Split into simpler sentences: The principal and most obvious goal was the development of a replacement aircraft for India's ageing MiG-21 fighters. The MiG-21 has been the mainstay of the Indian Air Force since the 1970s.