To finance an airforce you need billions of dollars, it doesn't come cheap, the economy in Pakistan has been shattered since the 90s militancy has made things and continues to make things a problem.
Here are the issues
1.chinese fighters like J-10,JF-17 JH-7A are completely unproven in combat the F-16s are better then all 3. Chinese technology is completely unreliable.
2. The PAF seems F-16 orientated because its a proven platform. Its hard to think of a decent Proven 4th generation fighter plane in the market that can offer what the F-16 offers.
3. The US and India is a big problem, These Bastards literally have hundreds of F-16s Rotting in Navada desert, if they were good allies they could literally sell several squadrons to the PAF at a throw away price, but the sons of bitches dont want to hurt indian sensitivity.
I wouldnt even call the current airforce Pakistan has defensive, the PAF only has 70+ F-16s against IAF 300+ SU-30/MIRAGE/Mig-29, to be able to defend your skies you need atleast 200+ F-16s, I dont think the IAF considers the JF-17 a threat, even though its an accomplishment for Pakistan, Its role is a supporting role. The JF-17 IMO doesn't pack that great a punch