So many things to buy for PAF anyway with L-15 coming in on some news. Budget are finite, cannot get and train for all the goddies at the same time anyway even if in an utopic world it would be available in 5 years.
I think this is part of the foundation they said they have started laying.
Once there is a steady stream of pilots going from L-15 to J-10, once the J-10 fleet is expanded, once there is plenty of alloy and engine capacity whether is be WS-13 or 19, then the PAF will be able to rapidly receive the J-31.
I think they also might need experience of the engine they are going to use, so we could see, say, WS-13 going into JF-17.
Once they receive them, the next pilot cadets can go directly from L-15 to J-31, this can save a lot of money and time.