While China has made amazing progress in all fields, including military aviation which is the subject here, not all is/was 100% perfect, the J-35 being unfortunately an example (hopefully rare) of that. Imo it was a mistake not to pursue the J-31/35 program with the same aggressiveness and determination as the J-20, which would have meant TWO stealth fighter lines boosting PLAAF/PLANAF, thus significantly improving their capabilities relative to the americans and their F-22/35 combo.
The J-31/35 should have been in series production by 2020 at the latest, and even a relatively sedate (by chinese standards) yearly building rate would have meant 250-300 airframes by now, complementing the 300 or so J-20s estimated to be built so far. This would have also meant faster retiring of the remaining J-7 and J-8 airframes which despite their upgrades can't be expected to contribute much in a potential conflict. And not forgetting the potential export orders that might have been possible if J-31/35 was in service already.
This would be a blunderous move back in the mid to late 2010s. As others have mentioned, SAC had much more on their plate. Their original competitor to the J-20 was said to be a "flanker" based 5th gen design and not the LockMart based J-35 design we saw in the FC-31 which itself was a flying advertisement (beats the Su-75 wooden mockup advertisement)/ tech demonstrator and nothing more until late 2010s when J-35 prototypes began showing up.
The reality is that the USAF and USN only have one mainstay 5th gen fighter. The F-22 at 180 or so units is just far too small a number (for the USAF which traditionally gets closer to 1000 airforce units for each type of fighter they've developed since the Cold War). The F-22 cannot be considered in the same vein. It wouldn't be a case of producing J-35 to match F-22 and F-35 combo because there for every F-22 there will be close to 10 F-35.
If PLAAF went down the path you suggested and rushed in the J-35, it would be a far inferior J-35 product that would be in service around 2020. PLAAF, PLAN and SAC took the time to determine what they need in a second 5th gen fighter. Rather than approaching this task with the mentality of rushing in a second 5th gen fighter regardless of addressing how best to combat US 5th gens. There is no doubt the current J-35 is a superior fighter than what a late 2010s J-35 would have been. If nothing more than having a better understanding of 5th gen fighter tactics and experiences tussling with the USN in the western pacific. Not to mention being more modern but i digress.
The real question is how much longer J-16 production will continue? Yes, very capable and versatile but strictly 4.5 gen and there's an opportunity cost to building it vs J35s. Obviously J-15T production will continue a bit longer.
J-16 is an entirely different beast. It is the product of J-11 series evolution and therefore the equipment, expertise, maintenance and parts supply chain is all well and truly reliable. It costs much less to build and maintain a J-16 compared to even a J-20 (i know duh but from a purely manufacturing pov ignorinig generational material cost differences). There is an advantage in being able to spam build J-16s with relatively little required attention and effort. As things are, it's almost a side business for PLAAF and SAC. Despite this, I don't mean to belittle the J-16. It is serving as the main PLAAF EW platform, that is important and until 6th gens with dedicated EW units come out or a J-20 EW variant is ready, J-16 production line remains.
J-16 also has about 1.5x the air to air combat load of the J-20 with similar level of "compromise" to performance ie not a fully loaded J-16 since IWB of J-20 full missile load has significant advantages over a fully loaded J-16. That's nearly a 2x improvement of A2A payload compared to J-35. Sure with even better networking cooperative engagement these days compared to the earlier generations of PLAAF CEC, J-11B and J-10 models easily fill that non VLO missile truck role as well as J-16 (not counting payload wrt J-10), but we don't build new J-11s anymore. J-16 is the Flankerest of Flankers. It is the F-15EX of the eastern hemisphere. It is
the eastern 4.5 gen heavyweight. Su-35 would be chewed out by J-16 in A2A and A2G. The only lament (if you can even call it that) from PLAAF on J-16 is that they'd love it with AL-41 engines aka any super heavy thrust engine ... that comment from PLAAF came well before WS-15 modernised version and in the context of comparing J-16 and Su-35. The comparison was even more damning on Su-35 than most of us expected... A2A difference between the two is like modern F-15 vs East German Mig-29.