There's an element out there that argues for copying even when its superficially only the outside. Why are there knock-off designer handbags and a market where people will buy them? It's because they want to be seen as successful. The people who make these bags aren't aiming to make a better bag. In the case of the Chinese military industries, they have to prove themselves to a government who has legitimize themselves by giving people examples of success which in turn are their successes. Might not be monetary but in the end but same mental pay-off.
The rationale for wanting copied handbags is different to the military wanting "copied" fighter aircraft or helicopters.
Handbags, shoes, phones -- they're copied for their status symbols and trying to fake a brand.
Fighter airframes, helicopters, -- they're copied for their proven utility rather than their superficial appearances.
People say China's military copies. Superficially one can make that argument. Has anyone ever established every thing inside is a copy as charged? Is China's HJ-12 an exact copy on the inside of the Javelin? I suspect the components are different. It looks like the Javelin because that's the point to make it look like China has achieved a Western level of technology. Unlike aircraft where proven aerodynamic shaping is important, they didn't need to make the HJ-12 look nearly alike to the Javelin. Everyone outside will see it as a Javelin copy so why do it when it's going to be criticized? It's because this is all about conveying to the Chinese people that the communist government has achieved near parody to the West who the public sees as unarguably successful.
People call HJ-12 a javelin copy because externally it kind of looks the same. (One could say that for Type01 LMAT, Hyun-Gung as well)
They say that for FC-31, Y-20 too, even though they're all clean sheet designs. People can say that, and they would be wrong. Clearly they are all clean sheet designs, only sharing similar configuration to other products.
But I think you're also wrong in saying that the military is specifically choosing some designs because of a need to demonstrate that they have reached parity with the west. I've seen no evidence of that, and as an observer of the PLA I see them choosing pragmatism rather than superficiality. (Besides the HJ-12 missile looks a lot like Spike, MMP, Hyun-Gung, but I digress)
IMO you are way overestimating the weight of what original/unoriginal means to the PLA.
IMO it doesn't mean anything to them.
For some required urgent capabilities they go for developing proven designs, for some capabilities they go for new designs if the technology is there. It's pragmatism, common sense.
Why are they still developing the flanker airframe into J-16 and J-15 when they clearly have the technology to develop their own original native airframe of similar capability? Because it would be stupid to expend resources on a new airframe design when they have a proven one that can fulfill their needs already there.
Why are they going for Z-18, Z-20? Because the PLA needs helicopters urgently and if using a proven design can cut a few years off the development track then that is worth it.
Why are they continuing to develop the Y-9, derived from the An-12? Because the An-12 is a proven, rugged airframe, and they can use the same airframe but with new materials, avionics, and engines, to make a good aircraft.
Why are they going for Y-20 when they could just copy the Il-76 instead? Probably because the PLA have requirements that can't be fulfilled by the Il-76's airframe, such as the volume of cargo and weight, handling, etc.
Why did they develop J-20, when they could have tried to copy F-22 instead? Because the PLA had requirements that only J-20's configurations could fulfill. SAC's triplane offer was rejected for a reason.
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Basically, what I'm saying is that the military does not choose to develop copies of airframes because of "inferiority complexes". Nor do they choose original airframe designs because of wanting to demonstrate a "payoff" or that they are now "successful" or whatever.
They make those decisions based on capability and urgency requirements, the technology available to their industry and the cost of choosing one road versus another.
