It's certainly unusual, but not necessarily suspicious.
When the Dark Sword first appeared some ten years or so ago, it was definitely a curious thing. Lots of us suspected if it was just a concept, a paper plane.
The fact that it then disappeared for so long without any photos or rumours about it being in development is not necessarily suspicious -- it could just mean SAC decided to pursue it internally in a subdued or even clandestine manner. Perhaps the PLA had even picked up on it and decided to do feasibility studies and further R&D into the concept, knowing that it would likely be many years until such a project reached any fruition that would present any kind of viable operational capability. It would be quite reasonable for the concept to go "missing" if that was the case.
Having the concept reappear in the form of a full size mock up -- well, that isn't suspicious either. Certainly unusual and a surprise, but not suspicious. If they have been doing R&D into the concept, the ducks might be lined up in a good enough row that the project might be moving to a next stage, and the mock up could be related to that. Overall, disappearing for ten years and then reappearing isn't too much of an issue.
If the UCAV was funded by the PLA it won't be appearing in an airshow.
If development continued, it could be doing so by private funds.
As for "unaccepted PLA projects shown at airshows" -- well, not sort of, but not really.
Yilong/pterodactyl/GJ-1 was first shown at Zhuhai before it entered service with the PLA.
Models of the first prototype of Soaring Dragon UAV, as well as the 611 CAC global hawk like drone, were both shown at airshows before emerging in real forms.
We had small scale models of both Sharp Sword UCAV and FC-31 ("F-60") at one of those UAV tournaments around 2011 or 2012, be shown off, before both of those eventually made debuts in full size forms.
So IMO there are certainly some products at airshows and expos which are reflective of PLA rejection. But there are also some projects and concepts that may actually end up being developed and pursued and eventually enter PLA service.
Certainly, quite a few of the UAV concepts and projects shown from 2006-2010 seemed to have emerged from plastic into full sized flying platforms, some of which are in service in some form.
It's not stretching the imagination too much to argue that it would be reasonable for Dark Sword to have followed a similar trend, but over a longer term.
Wing Loong had to prove itself live or in the export market and in the service of export customers before it was considered by the PLAAF. It was a roundabout means to doing it. Other weapons systems maybe trying to do the same, ranging from ships to vehicles. Its a risk that these companies have to take, financing the program as a private venture and hopes the PLA will buy into it. You do have this risk that the project won't be accepted and investment not returned.
This may come about due to the technological conservatism of the PLA brass who grew up on the core idea of the People's liberating power as the transformation power of the world, emphasis on the People. Then suddenly you have this technology that is displacing the People. More than a challenge its a disruption of religion.
What Dark Sword feels to me was that the project was canceled, perhaps around 2009 to 2011 as the J-20 began to show promise and the budget diverted there. Then when the UAV market is taking off, and the PLAAF has changed their minds about the issue, more liberal thinking towards UAVs, it was time to resurrect the concept.
Because funds comes as a finite resource, defense industry, along with their champions in the military, tend to be divided into factions that support one weapons system over the other. A good time of this was during the Carter Administration when you have the B-2 vs. B-1B vs. the B-52 upgraded with Tomahawks. Today, those who advocate for flight manned weapons systems might see UAVs as a threat to their jobs, and more than that, perhaps an abomination. I would expect much resistance within the PLAAF brass and ranks on UAVs. Given this, and so much going on with the energies and focus on the PLAAF acquisition and integration of new weapons systems --- J-20, J-16, J-11D, J-10C and so on --- one can expect a natural resistance to this technological disruption. The best route might be to develop it privately or maybe to get people with clout from the Party to get involved and fund the project, then try to change PLA minds by showing its feasibility and promise.
This may also be an indication that the technological capabilities of the Chinese defense industry is outgrowing the PLA.
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Forgot to add the link to the original picture in the previous post before it was PS'ed.