This is a rather flawed observation on here. Logistics and supply lines are not wholly constrained by locations and geography, even if the 2 artillery systems were employed on the opposing sides of China it will still put a strain on the factories manufacturing them largely due to fluctuations of demand of munitions that will most certainly happen.You're assuming that the 2 artillery types will be used in the same area. So that requires a number of additional assumptions.
Even if it does not, there is still the matter of daily expenses and maintenance of the guns and munitions. With both systems being in place for nearly 30 years side by side there is bound to be a certain cost incurred. Nor does the lack of prospect of a high intensity war makes standardizing equipment to save money in the long run and improve effectiveness irrelevant.1. China will actually get into a high intensity land war where it will be firing artillery shells. That is highly unlikely to happen.
Air superiority has been very much overhyped. It is something that is good to have, but in war (especially one against a peer opponent) it is nearly impossible to establish for any significant period of time. The even the most optimistic calculations would see that one side merely denying the other any form of air superiority. Nor can it be established everywhere at once. In this situation standardized artillery is most invaluable. And I not saying that China must switch to the 155mm, as they could have easily adapted any technology they gained from the GC-45 into the existing 152mm caliber.2. That the resources used to standardise on 155mm shells is worth it. Given that air superiority is a requirement to win a high intensity land war against a peer opponent, China needs to devote more resources to air superiority first rather than standardising on 155mm.
It is actually 400 PLZ-04, plus an untold number of PLL-09s (and various other 122mm caliber spgs) and the Type 83 152mm SPGs that are still in service with second priority units. All in all China's SPG numbers are definitely not "less then 400 SPGs" I can understand the justification of the 122mm calibers. But again there is no real reason to have have the 152mm caliber in towed and spg form alongside the 155mm. And again I will reiterate this point : China could have just as easily made the PLZ-04 in a 152mm form but consciously choose not to do so.Note that the US Army has almost 1000 M109 155mm Paladin SPGs, whereas the Chinese Army has less than 400 SPGs
Again refer to my post regarding point 1 and 2 above, this is the exact logistical strain that I am talking about. Why must the SPGs and towed howitzers use 2 completely different different type of ammunition that has no justification other than to deny its use in the other system ?3. That the different shell calibres will be used in the same area. More likely will be mechanised offensive divisions with 155mm SPGs and then non-mechanised defensive divisions with 152mm towed artillery
China already took the trouble to built a production line for the PLZ-04 and has so far already built 400+ of those things and an untold number of munitions of guided and specialized form . So the volume is already there. Plus they are also offering a 155mm for export. And they have just recently built a wheeled variant of the PLZ-04 so they have made very effort for the 155mm to be a signifcant part of their artillery caliber. Nor are they making any newer 152mm towed howitzers. So those production lines are just lying idle.4. Despite what you think, creating specialised tooling is actually expensive and only justified when you have sufficient volume.
So there is that justification.
And again I will say this, in the long run standardizing production of the systems will save more money than just plodding along with these 2 systems.