Even the Vietnamese news are criticizing that the Chinese are too secretive. They showed the J-20 since day 1 but not so for the Su-35 until now. While the Russian were so eager to announce every step of the deal. That made us have to guess and doubt if the deal even exist!
I don't think they are being secretive. It makes no sense for them to be open about their most advanced fighter (the J-20) but secretive about the Su-35.
Additionally, the Russians have been taking their Su-35 to air shows for years. There must be thousands upon thousands of Su-35 photos. Many people, including civilians and military personnel, throughout the world have actually physically examined the plane when it showed up at air shows.
There is literally no point in preventing people from taking photos.
In my opinion, it's the combination of "being a little ashamed" and "simply don't care". After two decades of developing their own domestic fighters and having the J-10 and the J-20 to show for it (plus the J-11/15/16, etc), it would be a little embarrassing to still import the Su-35.
Secondly, since the Chinese have been looking at Su-35-like planes (Su-27/30/33 and J-11/15/16) for the past 20+ years, it's simply not interesting any more. Thus less motivation for wall climbers. Especially when China has tightened up their security, which means more severe penalty for wall climbers, it simply makes little sense to risk so much for something that millions have known and seen for years. Then no wall climbers = no photos.
