Although we sometimes refer to certain fighters as flankers, they are actually completely different from cavalry. The cavalry is powered by horses, and these animals will tire and be blocked. The J36's jet engine will not feel any fatigue, nor will it hit objects other than the air. It only needs a slight deviation from the angle and it will blow away from the opponent like the wind. Passing by ten kilometers away, the lance named PL15 pierced the opponent's heart without delay. You can come back a few more times if necessary.To me it's pretty clear that a frontline fighter is still needed.
J-36 is like a mounted knight that can flank around or charge out from reserves during a decisive moment. Just like those, it is vulnerable if caught in wvr and unable to use it's superior mobility to disengage by a much larger force of men-at-arms/peasants (5th and 4.5th gen). Since after winning a few dogfights, the large plane will lose so much speed that an IRST equipped legacy fighter can just go up to it and hit it through its stealth and EW "armor".
The weakness of shock cavalry is that it can't be fielded in the same number as enemy foot infantry, even if they can nearly always beat them 1v1 and offer unparalled strategic options.
So China still needs its own line of contact foot infantry equivalents, which would ordinarily be J-20s, J-35s, J-16s etc. Here is where the J-XS(50?) comes in, it's like having a dismounted knight fighting among the ranks, bringing the J-36 protection level (stealth + EW) and superior fighting power into the main contact line.
When two lines of legacy fighters clash, having a few J-XS mixed in lets them mow down the opposition with ease unless the enemy also has their own dismounted knight equivalents.
The J50 will only complete its own job, such as slashing the opponent's F35 from a lower height.