or it’s because the stocks are available at that time and might as well use it. probably near end of service life missile.

Just doesn’t sound right… These planes are in a combat theater with a high potential of engaging in combat, not just against Syrian jets but, if things really go sideways, Russian jets. In a scenario like this, do you really want to remove a state of the art missile like the 9X and replace it with something 20 years old?