What is import substitution?
Exactly what it sounds like, substituting foreign systems with indigenous one, however it does not you substitute a 40 year old and obsolete foreign crap with an equally obsolete indigenous crap.
What necessitates import substitution?
Many things like sanctions, saving forex, reducing dependency on foreign suppliers etc however none of these reasons should restrict us from attempting an upgrade wherever possible, specially when you are going to need that upgrade few years down the line anyways.
Import Substitution is not upgrade
yes, Import Substitution is not an upgrade however it gives us an opportunity to upgrade, specially if we have the foresight for it
Its for increasing maintainability of legacy(in our case Soviet or British or French) assets while decreasing capex due to saving on forgien exchange
Remember when we replaced alloys used in our Soviet engines, we not only replaced them, we upgraded them with a better quality alloy, why because that replacement request opened up an opportunity for DRDO/MIDHANI to do more than just 1:1 replacement,
A similar thing could have been attempted here, if DRDO/IAF had the foresight.
Not every legacy asset can go under major upgrade.
Upgrading to a more modern HMDS is not called a major upgrade, you know what would be called a major upgrade, upgrading mission computers or engines or radars would be called a major upgrade.