You dont just buy an aircraft, you buy a whole set up that supports the aircraft
How many J10's would we have bought?
We would require extensive pilot training taking many years
A whole scale training of many personel to deal with the J10B
Infrastructure to support the J10B
It takes many years for air forces and its pilots to master a plane, come up with an effective training and flying doctrine
The above is just a small sample of the issues involved when inducting a new plane, now why would we do all that just to induct another 4+ gen plane that would not give us an advantage over the F16 (except maybe it would be a chinese plane not subject to sanction)
With the F16 we have everything already in place, from very experienced pilots to pit crews and infrastructure. We need to expand our F16 fleet to in an ideal world around 130 planes, Our relationship with the americans is improving and their will be thousands of second hand F16's on the market going very cheap. We can buy cheap and MLU if we need to, we have top line pilots with years of experience and some top line weaponery.
Then concentrate on improving the JF17 Block 3 into a worthy fighter, JF17 is a new fighter and look how many issues we are having inducting it, it takes time to train the pilots and mechanics and come up with a doctrine so the JF-17 wont be truly ready until maybe another 3 years. Once it is up and running with aesa, irst then it is good to go
Our next 5-10 years should be spent on ensuring the JF17 is as lethal as it can be and to inducting 150-250 planes, with the option of more this will be our work horse figher for many decades. Adding another 50 F16's to our inventory is vital
We will then have the time to plan and save money to induct a 5th gen fighter which will be bought in batches and gradually built up