Personally i beleive, IN & Mod should Some how works out to convince Japanese to sell its soryu class sub to India. The soryu class is no longer cutting edge for Japanese, they already building its successor boats to soryu. So Its up to Mod how they can convince Japanese to do that, why should they keep their second best sub exclusively to themselves when they are building more advanced boats, why not they are sharing with a quad member.
Lastly,The one thing we should learn from Japanese is that how they are keeping their sub surface fleet relevant always, no matter how advance the boat is they will start producing successor to existing one in every 15 years so. Advantage of this strategy is Their navy used to get brand new sub , their domestic industry will never out of order and experienced shipyard crew never get wasted & R&D is constantly get upgraded.
Even UK is having similar ship building /designing strategy. Royal Navy will not use its surface fleets more than 15-20 years( except ACs). On the other hand, we recently decommissioned INS Rajput after 40 long years of service, and still three more ships from same class is in service with IN.