China's civillian production numbers for pretty much everything is several times US numbers. So, if they were serious about military production, they can easily produce 300 fighter jets per year, produce whole lot more nuclear and non nuclear submarines along with more surface destroyers and cruisers, they can also produce enough tanks and other armored vehicles to completely replace all the old models.
1.4 billion population is certainly not a problem getting more recruits. Training can be expedited as well. Besides, its not like China is going expand the military. Just replacing old hardware with newer stuff much faster is not going to need more recruits.
The problem is the will to spend more. I always say, China is still in hide and bide mode when it comes to the military. They are still worried about the optics of appearing too aggressive and try to show how they are not raising military budget that much.
And how much money is needed for maintenance and pilot training? After a couple of decades when the airframes are old, what do you do with them?