If PAF had done that, they'd be taking down planes in Indian air space, not Pakistani - Pakistan would have pretty much declared war on India for missing a strike in Balakot. It'd help to keep the context in mind. This would not have helped our case, at all. None of the Indian air crafts crossed into our air space that day, other than Abhi's.Not possible due to two main reasons:
1- Loss of Indian Air Superiority. Shooting down 12-planes would have meant a lot of confusion and disorganization within IAF, giving CLEAR ADVANTAGE TO PAF. PAF would have decimated Indian Army and navy.
2- Nukes specially tactical nukes.
And PAF while could maintain air superiority for a short time, would not have been able to do the same in a stretched war and we'd have seen losses too - had this turned into even in just an air war. And all for what? One strike, that did not hurt anyone in Pakistan?
War is a fairly serious business, our economy took time just to recover from the expenses of long-drawn CAPs that were flown in the aftermath of this incident. We'd have never afforded a war, our economy has been in shambles for a good while now.