It's a surprising decision. DRDO is working on maritime surveillance technologies. But it's unclear if they have reached a point where the P-8I can be replaced this quickly. The assumption was it will happen after 6 more are ordered. The US had also cleared the sale last year.
At this time, DRDO is working on a C-295 based MPA called MMMA. And the technologies will be tested on an Airbus A319. At the very least, they have radar and EO/IR technologies for this role. So I guess once it's tested on the A319, they can scale it if necessary.
Anyway, the navy is also making changes to their fleet. They want more unmanned surveillance than manned. This goes back to what I was saying about manned ISR becoming irrelevant. Too expensive, too risky and less endurance. So we may want more Sea Guardian drones, perhaps even the Triton. I'm more inclined to believe this to be the case. Each Triton costs more than $200M.