As a general rule, the more capabilities you want to fit into a fighter, the more time consuming it gets. That is because as you pack more and more stuff into a set space, you find you need to shift existing parts around to make space. Often even that isn’t enough and you end up with physical extensions to the airframe itself, be it in the form of CFT, lengthening or avionics spins etc.
The BLK3 upgrades would naturally take longer because not only is it a much bigger increase in capabilities, you also have the existing blk2 upgrades having taken up much of the original available space.
Relaxed FBW restrictions to make the jet more agile could also require additional airframe strengthening, as the previous restrictions weren’t just put in place purely to nerf the plane for lolz and giggles, but rather to keep it within safe operating parameters.
I think the BLK3 will pretty much exhaust all existing airframe upgrade potential, and any further upgrades will probably force the plane to go the F16 route by expanding airframe itself to make room. But at that point you might as well have gone for the J10 as you are pretty much reinventing the wheel to try to achieve J10C level capabilities, but still not quite get there due to the airframe and engine output restrictions.
But I think the PAF is resisting the temptation of the J10C because they want to save up for the J21 or other Chinese fifth gen that will decisively outclass Indian Rafales and give the PAF the kind of technological advantage it needs to hold its own against the IAF’s numerical superiority like during its golden age.