Take a good look at the following graphic:
There are different methods to detect submarines while submerged:
1. Multi-mode radar
2. Sonobuoys
3. MAD
You are alluding to ASW range of OPTION # 1 but overlooking other sensor systems in your calculus - ASW range and capacity is DYNAMIC and shaped by employment tactics for the needful.
How Sonobuoys work?
Just one P8I can deploy over 100 Sonobuoys for the needful.
NUMBERS GAME
Indian Navy P8I fleet strength in the present = 8
They are receiving 4 more in this year.
Multiple aircraft = ASW mission made possible on a much wider scale and capacity with exchange of information in the mix.
USN Factor
Should Indian Navy feel that its ASW missions are being constrained by Pakistan Navy through relevant countermeasures and tactics, Indian Navy can reach out to USN to facilitate its ASW missions (COMCASA). Imagine this.