There are over a thousand fighters which can fire off the PL-15 but (probably) hasn't integrated the PL-16 yet. Unless the PL-16 is a significantly superior missile, it's possible that they never get upgraded for that capability. In any case, the supply chain for the PL-15 is well established, and the automated factory making them should be able to churn them out very quickly, so I suspect they'll stay in production for years to come.
First off, the "established supply chain" argument can be applied to any obsolete weapons, but you need to replace them at some point. If PL-16 is better in every way, why would you not supercede PL-15? Perhaps in a WW3 setting you might not have the time to make the shift and need to make as much as you can. But we are not there. China can afford to reduce production a few years for shifting to a significantly better weapon, and retool the PL-15 production lines into PL-16. This is the better decision in the long run.
Second there is no reason what can fire PL-15 cannot fire PL-16. Their dimensions are compatible, and we already know some fighters made the transition. And if they somehow cannot (I doubt it), there is still left over stockpiles of PL-15, no one is throwing it away.
Admittedly, it's amusing to think that there can be serious talk about retiring a missile that's only 10 years old. Even more so when the only normal (so not counting the AIM-174 and R-37M) foreign missile that can match it is the AIM-260, and that one isn't even in full production yet.
No one is saying it is retired, it will still be in inventory just not actively produced. Mind you PL-12 is made 10 years before PL-15, and was superseded by PL-15. It is nothing unusual to replace missiles designed 10 years ago.