Good thing you mentioned that. I took a look at Scramble ORBAT for both PLAAF and PLANAF to see which ones are still using J-7s and J-8s. I ignored the training brigades and non-fighter brigade (like J-8FR ones).
From that list, I only got 2 J-8 brigade/regiment left. 78th brigade with PLAAF and 15th PLANAF regiment. Since the new flanker deliveries for 2nd half of 2021 are still making their way into service, it would not be surprising if both of these regiments are gone by the end of this year.
There were definitely more J-7s around. I had 12 brigade and regiments.
J-7Gs
44th and 52nd brigade
J-7Es
86, 88, 97 and 132 brigade
20, 53rd air regiment
J-7H
57 and 63 brigade
J-7II
71st air regiment
J-7L
21st brigade
Due to less attention on these units and year end deliveries, it's possible that fewer than this many brigade/regiments are actually in service by the time the shuffle of equipments are completed. It's also possible that 1 or 2 of these units just go away in the next couple of years. If we assume that the average procurement level of PLAAF over the next 3 years to be 45 J-20s, 40 flankers and 25 J-10s, it would be capable replacing about 3 to 4 brigade/regiments a year. The J-7s could very well be all gone over this time frame. That's also factoring in some of the older Su-27/J-11 units getting replaced by J-16s.
I think your underlying assumption is that the PLAAF will follow the trend set by western air forces and reduce numbers as it moves up generations. But I don’t think that will necessarily hold true.
The western air forces cut numbers because they had built up an excessive number of fighters during the Cold War which were not needed anymore after the collapse of the USSR.
China doesn’t have such a historical dividend to draw upon, and more critically, is being actively targeted and engaged by the US for a new Cold War.
Under that kind of threat, it would not be unreasonable for China to want to increase its fighter regiment numbers rather than decrease them. Just look at what is happening with PLAN numbers. Not suggesting the PLAAF will see anything like as dramatic an increase in numbers, but think the odds of an overall increase in PLAAF operational units and aircrafts is much higher than us seeing any reductions in the foreseeable future.
As PLAAF budget continues to grow, it will be better for PLAAF to accelerate J-20 procurement rather than keep ordering more J-10s. After all, why max out at 50 a year? Why not keep investing in your production capabilities to do 70 or 80 a year?
Yes, it will be more efficient in peacetime. But sadly I don’t think China can now afford to assume that they will continue to live in peacetime going forwards. At least not unless and until it has reached such military and economic might that the US is forced to permanently take the military options off the table.
So why not both? Indeed, I see moving the J10 production lines as preparation for precisely this, that CAC will now be able to focus on massively increase J20 production while Guizhou maintains J10C production.
If anything, I see J16 production ending way sooner than J10C, as SAC also ramps up production of J31/35s and keep the flanker production skills alive with J15 deliveries.