What would be the pro and con of having a single type of advance trainer jet (JLXX) vs having 2 types (JL10 + JLXX) or 3 types (JL9, JL10 and JLXX)?
In my opinion the JL9 was developed because it was lower risk vs JL10. It also used a Chinese made engine instead of an imported one.
Without solving the engine issue the JL9 won't be retired.
You could also argue JL9 is a better platform for training single engine jet pilots. Like for the J-10.
The JL10 more accurately models the flight caracteristics of the twin jets like the Flankers, J-20, and J-35.
I expect the JLXX to be developed to be able to train carrier pilots. Notice the double wheel nosegear.
I don't know, although I only saw the JL-9 as a transitional solution anyway, and the current Chinese system is even: CJ-6 - JL-8 - JL-9/-10 - J-10AS/J-11BS the FAs ... I'm afraid the question should rather be, why does China still not have a modern turboproptrainer?
Lack of engine.
Therefore, I think the JL-9 as well as the JL-8 will slowly find their end so that the JL-10 will still change to CJ-6 to the JL-xx after the first basic training. I think the first part with CJ-6 & JL-8 should be replaced in the long run with a modern pattern like TA-20.
Retiring the JL-8 might be a bad idea. The Russians are now developing a replacement for their single engine jet trainers. They are cheaper to operate than the twin engine trainers. Italy is doing the same thing with the Aermacchi M-345.