I for one do not see an end to the Block 3. PAF logically will want most, if not all, its air-defense and strike a/c equipped with an AESA radar. There will be some retrofitting on the block 2 airframe but by and large, the force will keep on producing block 3 to replace older aircraft still in service.
The current strength of PAF includes 44 F-16 AM/BM (All MLUed a/c) 13 ADF (Procured from Jordan) and 18 Block 52. Roughly 75 a/c. 25 J-10C out of an order of 36 aircraft. 47 JF-17A Block 1, 62 JF-17A Block 2, 25 JF-17B Block 2, 15 JF-17A Block 3 out of an order of 50 a/c. That totals 249 aircraft. with further 9+35= 44 aircraft on order.
IMHO 231 aircraft have to be replaced (this includes Mirages 3 and 5 and F-7PG's).
As per my calc. following aircraft will be replaced by:
80 Mirage 3 various variants will be replaced by combo of Block 3 and J-10C (I suspect a D version of J-10 might come into play some time later)
90 Mirage 5 various sub variants will be replaced by Combo of JF -17 block 3 and J-10C.
53 F-7PG's to be replaced by JF-17 Block 3A's.
The above calc. if correct, means that 231 a/c procurement is in the pipeline for the next ten years if we keep all F-16 flying till then. If nothing else the PG's will be replaced by 60 JF-17A's B3 with a further 25 aircraft to be ordered (35+18=53). So, we may see a further 18 aircraft being ordered of the A or B variant of Block3.
This leaves 178 aircrafts to be replaced. Some folks are reporting an order of 50 J-10's instead of 36. This gives us a further squadron of J-10C's that are being earmarked to replace one of the mirage squadrons.
The bottom line is that for the next ten years PAF will be procuring 178 aircraft to replace its aging fleet on a one-to-one basis. JF-17A or B, Block 3 will play a definitive role in the procurement.
KAAN, J-20, or J-35 types will be on another plane and will most likely be inducted in replacement of F-16 Fleet.
That is my 2C worth.