There is another key difference between the J-20 and the J-31 which is price. The J-31 would be much cheaper than the J-20 and therefor you can produce more of them. Personally I don't think the Americans idea of a all stealth fleet made up of F-35's is a good idea. So China (and other countries) should keep the 4+ gen planes in their inventory and keep the production lines open. But a plane like the J-31 does have its uses and not only in terms of combat but also psychologically and politically.
Like how the F35 is 'much cheaper' than the F22?
The PLAAF would have paid especially close attention to that whole messy story, and so would be very keen to not make the same mistakes.
SAC's decades of underperform and the low regard the PLAAF holds it would also work heavily against the J31.
Simply put, the PLAAF lacks faith in SAC being able to deliver on time and on budget, so why would it divert funds from an already phenomenally successful project, from a company with an examplary track record, and from a clearer more capable design to gamble on SAC being able to break a habit of a lifetime and actually deliver what the they promised?
The extra cost, logistical burden and uncertainty over the project just doesn't justify any PLAAF investment given they already have the J20.
Considering how much it will cost to bring the J31 to operational state and the costs of setting up the production line and costs of making the plane, I really question just how much, if any actual savings the J31 might bring compared to just building more J20s short of ludicrously optimistic JSF like projected production numbers.
The F35 only made sense because the USAF wanted to go full stealth and wanted to sell it to everyone under the sun and their dog.
But even with such a vast production run, the unit price is still eye wateringly over budget, never mind the original sales pitch.
I think the most likely way SAC will end up building stealth fighters for the PLAAF is if they were given a second J20 line to run.
