My understanding is that one of the specific points is not just about the transition of the PL-15 line to an ARM version, but that PL-16 itself is a more capable and more affordable missile with better efficiency in production due to more modern facilities.
Sure they could go back to producing PL-15 AAMs at some point in the future if they need to, but.... why would they?
Yes, the PL16 is a better new model.
As for why they might want to go back making more AAM versions of the PL15 in the future, well the obvious reason would be a massive hot war against one, or even multiple near peer adversaries.
Sure the PL16 will be better and cheaper, but better and cheaper doesn’t mean you can make more of them faster when you run into production bottlenecks.
Having a different missile that uses different supply chains is an easy way to bypass certain production bottlenecks that might occur when cost becomes a distant secondary consideration to quantity output.
This is especially relevant when the PL15 itself basically outclasses anything operational deployed by the most likely opfor, so it’s not like you are making an inferior missile to what opfor might be using. As time goes on and more advanced AAMs gets fielded by opfor, that will change, but even then it will take a long time for the new missiles to build up inventories to become the mainstay of opfor forces, especially if China continues to choke key RE resources to opfor MIC. But even after opfor new gen AAMs have achieved critical mass, having a legacy mid-tier weapon that handily outclasses opfor legacy mid-tier equivalents is massively beneficial.
A slight separate but related rationale would be future mothballing considerations for currently in service combat aircraft and their wartime reactivation capabilities.
If you are desperate enough to be pulling legacy retired assets out of mothballing and reactivating them, odds are you don’t have the time, or potentially even the resources, to spare to give them a full radar and avionics upgrade to allow them to use the latest and greatest AAMs. But you also don’t want to be using decades old missile stocks if you can avoid it. So having the ability for re-activated airframes to use freshly manufactured missiles would have a massive advantage. Especially if opfor is in a similar position and using re-activated airframes themselves at that stage, but pairing them with expired and refurbished missiles.