The infrastructure that went into the F-16 includes decades of institutional memory, from personnel who understand how the platform works, SOPs, maintenance facilities, etc. It's an overhead that has at this point been paid for, making any net new addition to it more affordable than an entirely new fighter platform. Those new fighter platforms will come, but they are likely much farther out, and in the lead up the PAF will likely build on the fighters it already has: J-10CE, JF-17, and F-16. The only thing that matters for the F-16 is US willingness to sell, but otherwise, PAF is game for new-built birds.