Why does the PLAx want both L-9 and L-15?
Commonality of parts between the L9 and J7 is likely also to be a major factor.
The PLAAF operated J7s for a very long time, so would have built up a substantial and comprehensive logistical infrastructure for it.
With the J7 still operating, the PLAAF would be obliged to keep producing spare parts for them and keeping engineers and technicians trained up on them. With the L9 able to use many if not most of those parts, that is going to massively cut down on its operating costs compared to other types that share no common parts with other in service jets, and also provide the PLAAF with an easy and cheap way to re-task existing engineers and technicians, who might otherwise become surplus to requirements or require comprehensive retraining as it winds down its frontline J7 fleet.