Well,we only know about it for 4 years now.And for Chinese members,the "Skepticism" has a touch of hyper nationalistic attitudes as well.
By the way,your claim again has brought us to the same point,if China is making "Flanker Derivatives" which is better than Russian one,why they bothered to by SU-35 in the first place??There is no doubt though we'll see a Chinese version of SU-35 in near future even with all the Russian safeguards in place.
My perspective has always been this: if the Su-35s were truly meant to supplant or even complement an existing Flanker (secondary to a technical deficit or whatnot), the Chinese would have purchased far higher numbers of the aircraft than merely 24. Additionally, we would see much slower production & development of domestic Flanker variants, which has not been true (the J-11D, J-15, J-16 are in full production or development).
From a technical basis, the J-11D and J-16 (and also J-15A to some extent) has numerous enhancements such as AESA radars, application of CFRP composites & RAM, additional hardpoints, ECM/EW upgrades, etc., many of which are simply not found on the Su-35.
There are a few theories on why the Su-35s were purchased, based on their camouflage and the general parameters of the deal:
1. Stop-gap until more advanced aircraft (J-11D, J-20, etc.) come into service
2. Salvage of its 117S TVC engines (unlikely given the development of a similar-performing WS-10IPE)
3. DACT or OPFOR (very likely given its camouflage, which reflects that of existing PLAAF "Blue Force" simulator aircraft)
4. Supplement inadequate J-11B/J-16/J-11D production (possible but it will cause logistic issues)
The Su-35 purchase of 2016 is very different to the Su-30MKK/MK2 purchase back in 2000s (which involved 100+ aircraft); it's very unlikely a technological gap was the main reason for the purchase, especially with the appearance of advanced Flanker derivatives as stated above.
