Ok - well F21 is a good evolution in fact an awesome evolution of dependable F-16. Looking at the history of Pakistan US relations, I can clearly say that it will come with lots of string attached and with lots of capital spend to have them. beside it will be quite a few notches lower than F-35 but better than anything IAf can fiel short of 5th gen fighter provided USA share with us the technology. using same argument buying j-10D or chinese version of su-27 will give us a platform not so expensive as F-21 but not as higher quality as F-21 also. it will be good enough to tackle rafael and IAF. No strings attached, cost effective. best solution is to jump into 5th gen now. for that the only solution that could become real in my opinion is some version of J20 rather quickly. all other options needs 5 -10 years wait time. However, if USA offer PAF F-35A. I would say jump on it. this will help project AZM many fold. it will be costly and would have similar kind of stings that we can expect with F-21 nut then again the technological jump is simply enormous.
Pakistan will have to Beg for F-35 or F-21 as USA do not consider Pakistan in the same boat as many other countries receiving top notch fighter from USA. we have been asking for F-16 for such a long time and when we are going to pay for F-16Blk52+ second batch it was derailed. and PAF was asked to pay for it out right. knowing very well that the cash crunch Pakistan cannot afford it. even though PAF and Pakistan followed the process agreed with the state department and Pentagon.