The original versions were not up to our requirements.We should had added J10s much before than.
Secondly we neither had finances nor the imminent need. PAF waited until our adversary committed to a $9B purchase and put its plan in motion.
With China committed to CPEC, it is in their favor to have a strong Pakistan Defense. Things have also accelerated with their confrontation with India.
I believe by providing Pakistan with ~ 40 J10, 45 J-15, (unkown JH7 qty) even for free (let's say $3B) investment on China's part upends the apple cart. Now India has to look after its southern flank and divert a lot of resources defending areas like Pune or Chennai or Kochi that they had never intended to.