There is another reason I mentioned before. J6 and j7 don't have fly by wire, everything is mechanical! To convert them to a UAV would cost more than the plane itself. Will need to strip everything out and put in a more expensive FbW system the plane is not designed for.This discussion is better suited for the UAV thread so I'll keep it brief.
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PLA watchers within China have written about this topic. Yes it seems like a good use of old J-6 and J-7, but that's because you're overlooking a very important resource: maintenance and runway time.
To keep a fleet of converted J-6 and J-7 UAV airworthy requires maintenance crew, particularly in the vast numbers often envisioned. You don't want them to be unreliable and blow up on the runway at a time when runway is really busy.
If forceful reunification actually occurs these UAV require even more ground crew time to fuel and attach iron bombs to them. And they have to line up and take off on actual military runways, taking up runway time that may be better off being used to send up actual manned aircraft that are also needed.
On the other hand, if you were to build some new dedicated kamikaze UAV you wouldn't design them like that, instead the result would look more like this IAI Harop here:
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While it still kind of looks like a plane you notice it's launched from a boxy launcher and comes in a sealed unit to minimise maintenance. It actually in many ways is more similar to a cruise missile than an aircraft. Given the above mentioned cost in manpower and runway time for converted fighter UAVs, it may actually be cheaper to just build new dedicated UAVs for this purpose.
So just build UAVs is much cheaper.