One to one replacement is impossible. Remember, we bought one of the cheapest aircrafts F-7s first in 1988 and then in 2002 to fill up numbers. These were 2nd generation aircrafts bought at the time of 4th gen aircrafts (F-16s, M2Ks, SU-27s, etc). So these were two generations down from the topline fighers of the time. Those weren't some cutting edge or "game changer" stuff even at their induction. So you can't expect such gap fillers / cheapest aircrafts to be replaced with 4.5 gen fighters. It will be ridiculously expensive to aim for one to one replacement.
Also have to remember there weren't any UCAVs 20 yrs ago. For airforce, these UCAVs are also an addition and a platform. It requires its own hangars, it requires it own pilots, their own trainings, UCAVs maintenance etc. So don't forget that resources being allocated to UCAVs are being deducted from airforce budget and those platforms will be taking important roles from manned platforms like for COIN etc or even against india as SOWs delivery platforms. So naturally your manned aircraft strength will reduce when you are pouring resources at UCAVs as well. BTW, this trend will continue, UCAVs will keep increasing and manned fighter strength will keep reducing. A time will come when we will have more squadrons of UCAVs and less squadrons of manned aircrafts but extremely advance.
"Over time", not only the older jets will be replace 1:1, our fleet strength will increase also. So in fact, we'll replace 1:1 + add more. I think we'll see a 450-500 jet PAF in the next few years.
All the planning and work being done is to take Pakistan in increments to a near $700 billion economy, then to $1 trillion and above. Our trajectory will be visible in 2 years IA.
Post 2028, we'll need to protect Chinese trade through Gwader and offer proper security to future Central Russian nations who want to use Gwader. For that, we'll need a dedicated air arm for the navy. That's easily 2-3 squadrons plus 180 replacements. Add near 50 combat ready ship (frigates, destroyers) for PN also! That's our next ten years pipeline.
So as the economy grows, funding for PA, PAF and PN will grow too. At a $700 billion economy, a 3% defense budget is $21 Billion. At $1 trillion, it's $ 30 billion. The size of enemy we face is massive (Indian defense budget is $80 billion per year!!), so our defense budget needs to be at least $ 20 billion or more to get proper capability. So the end result over time? The PAF will have near 450-500 Jets, all with latest gear and near 80-100 stealth jets included.
If the above seems "too much" because we've never seen economy grow before, take a look at Indonesia. Their economy is over $ 1.2 trillion. They are investing $10 billion into KAAN, have KF-21 partnership with Korea, operate F-15 EX, Rafale, F-16's, SU-30's, etc. Because they can purchase! We are establishing our own defense industry so we'll build ourselves, with joint ventures with Turkey and China after 2030 and sell to others also. So good and mature planning is in place IA.