The upgrade to the P-3Cs was carried out around 2005-2007 time. About 15 years ago. You contradict yourself when you say the P-8 has an edge over them yet did not detect our subs. This was due to PN operating the P-3C for 20 years, and IN being very new to the P-8 (as you state)
The upgrade was the Update II.5 level. Sub hunting tech and missile tech have progressed very far in last 15 years. The detection systems in the plane, the ECCM in the Harpoon (and it's relatively short range) mean these beasts are still effective but probably unable to hunt and find the latest subs and take on Indian ships beyond the range of many of their long range SAMs.
You can read about the history of PN P-3Cs here
Pakistani P-3(click to view full) Pakistan's location on the Indian Ocean next to the Persian Gulf, and its rivalry with India, ensure that its maritime patrol and strike capabilities will need to operate across a wide expanse of ocean. Maritime patrol aircraft are critical to that effort...
www.defenseindustrydaily.com
How feasible it is to maintain a fleet of just 7 (assume more attrition so could be 5 in 20 years) is questionable. Especially as we will need to replace many electronic systems from the US who may not manufacture them anymore.
Secondly, it is a very good bet PN MPAs may at one point be tasked with locating US ships and Subs near Pak coastline in the future
Finally why would PN be inducting Sultans now if we are planning on using P-3Cs for another 20 years, why have three types of MPA in service at the same time?