The Plan B, to be honest, is to take whatever money's left and save it for J-31. The PAF will not pursue any other option unless it somehow comes upon a miraculous economic surge, in which case it would gun for a 4.5 gen fighter to hold the fort down until the J-31 is ready. But unfortunately, that isn't going to happen, so it is F-16s or nothing. We have the JF-17 to basically fill in the gaps where it matters, and though the lack of a front edge fighter will be felt, it won't be anywhere near as painful as the weight Pressler left behind in the 1990s. PAF can cope.
But yes, it is insane how Pakistan plunging itself in an internal war and enabling the U.S to successfully hold Afghanistan didn't translate into it at least being able to freely buy surplus F-16s and stockpile them at will. Countries that do less get more. There are times where it might have been prudent (and still prudent) to actually point the stick at the U.S, i.e. seal the border and tell them they're on their own from this point on wards, maybe then they'll release F-16s