It's an option. Sanctions haven't stopped Pakistan from buying second hand F-16s, considering US sanctions are worse than EU sanctions.
Pakistan only needs around 2 squadrons until the FC-31 is ready for export, so it's not a huge investment to make.
getting more F-16s would be the most prudent course
the us allows pakistan arms sales during small windows in time when its mood changes and it wants something from pakistan
as soon as another window opens up, pakistan should try to pick up as many f-16s
think of it like the iranian f-14 issue, have a large number, and if need be cannabilize some to allow the majority to keep flying
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1. buy as many as possible
2. buy as many spares as possible
3. get the latest upgrades allowed; try to push for the most cutting edge tech allowed (such as sabr aesa radar)
4. buy all the support equipement to allow all the latest training and maintaince to be done in pakistan (i.e. upgrade the mirage rebuild/JF-17 production factory to be able to do major repairs on the f-16s; similar to the turkish f-16 factory)
and get all this as quickly as possible within the small window of a few years that us allows arms sales
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if the air force needs 26 squadrons of 17 planes each, and 14-15 squadrons will be the JF-17, and there is currently already 4-4.5 squadrons of f-16 in the fleet, the PAF needs 8 more squadrons of advanced 4+ gen fighters
getting 2.5-3 more squadrons of F-16s and 5 squadrons of J-10Bs (85 fighters) would prove a decent mix and balance the risk with induction of advanced Chinese 4+ gen fighters (this mix can be tweaked based on availability and price)
85 J-10B
100+ F-16
250 JF-17 (as long as the design is upgraded to take on agile fourth gen fighters)
this should be a good enough mix for the next 15 years till the PAK-FA and J-31 become options for the IAF and PAF respectively