We could buy at least 2 submarines in the same budget of that 8 subs we are procuring.
Reportedly 8 subs deal was whopping 5 Billion USD. Do you think this much money is laying around for next purchase ? Or you were suggesting that instead of these 8 we should have bought 2 nuclear? These 8 submarines will benefit lot more than 2 nuclear subs for Pakistan's threat environment.
Expecting Pakistan to buy nuclear subs is just wishful thinking. Previous Pakistan's submarine purchase was more than 20 years ago (Agosta-90bs). See the pattern, It will be atleast decades before Pakistan goes for next mega submarine purchase. Cherish whatever you are getting. With such constrained budget these 8 subs are already a great deal and will add lot of power & capability.
Nuclear subs are for the major powers OR for the nations where their area of operation is global or at very long distances. Those nation's have their adversaries at long distances, like US- Russia, US-China, China-India, China-Aus, etc. Pakistan isn' t going to challenge any far off nation ever. Its entirely focused towards India which is its immediate neighbor.
Another benefit of nuclear sub (other than range) is that it do not have to surface for air (recharging batteries). The AIP technology improves this at a great deal. Those subs can be even more quiet than nuclear ones.
Lastly, SSN (a nuclear attack submarine) could be an option at a very later date, depends upon economic revival. But SSBN (nuclear powered with nuclear capable BM sub) is not going to be an option for Pakistan (that's my opinion). Pakistan will have to be drastically changed in order to get something like that. Such weapons are for truly independent states / global powers which can make their own policy. States which have guts enough to stand for their sovereignty and not attack own govt for others appeasement, States which do not fear for sanctions, States which are independent economically. No one is allowing Pakistan to carry its nukes and roam around the world. Global order isn't extra bothered about Pakistani nukes because we have ensured them that these nukes are for india specific. The SPD ex-DG himself said in US one of conferences that Pakistan will not increase range of its strategic BMs more than what they are. He was giving a clear message that don't worry, our nukes are for India only.