Professor and former 624 chief engineer/designer Liu Daxiang said the WS-15 would achieve design certification around 2021 to 2023. If I'm not mistaken, mass production can only begin after that. In other words, it is still some time before the WS-15 can become operational. If the PLAAF believes that the TVC system is mature enough, then why not install the system given the Taihang's increased thrust and reliability? This would provide immediate maneuverability benefits to the J-20 and let pilots familiarize with TVC.
Essentially, money. Please understand, PLAAF don’t have infinite resources, and they still have mass 2nd gen fighters need to be replaced. J-20 is probably the most important project, but far not the major one, there are still tones of other places need more money to go in.
Well, that is the big picture. So from a slightly technical angel to understand this issue is because TVC will still loss thrust, and j-20 at the same time is maneuverable enough for their current mission. If I only have dozens of J-20 in hand, why would I risk them in dogfighting with other fighters instead of doing what it supposed to and d the best, hit and go? Thus, given how little amount j-20 they have, why bother?
Also, follow the same idea, when ws-15 is out, j-20 turns in to mass production, more pilots will transfer from 3rd or even 2nd gen to it, which is a complete re-education process, and a good time to learn dog fight in TVC.
