Mate, do you think that these aircraft are like furniture that can be bought at a fixed price per unit and put in hangers and that's it? What about costs of development, training, maintenance, repairs, supply of spare parts, and weapons? The maintenance part alone will be a headache due to costs of maintaining increasingly sophisticated engines and extremely sensitive RAM application(s). Pakistan needs advanced infrastructure to maintain a fleet of J-31. Then there is the question of how good this jet is because China is lacking in development of stealth aircraft in comparison to the US.
Where is the feasibility report of this project? Where is the proof that this project will have no impact on Pakistan's fragile economic situation? The military continues to add to expenses while the country is facing sustainability challenges and borrows from others to make ends meet. A lot can happen in 20 years that might not suit Pakistan. India also has a convenient counter to this induction in case you forgot. You have very simple thinking.
Pakistan defense should be practical and put under civilian academic oversight to understand how it affects Pakistan's economic situation like in developed countries. Otherwise, Pakistan is well on its way to becoming North Korea 2.0 due to hubris of generals. But they seem to have a virtually endless supply of patrons to encourage them. Meanwhile Pakistan is losing bright minds to foreign countries. Keep it up.