Jesus Christ, why is this argument going on? I think it's fairly clear that the Su-35 has some advantages over J-11D and vice versa. To summarize all the valid points here, the PLAAF in all likelihood acquired 24 Su-35 in order to:
1. study the advantages it has over J-11D and combine them with the advantages that J-11D already has to take the Sinoflanker to the next evolution.
Except the timeline doesn't fit.
If you followed the long and tortuous development of this deal, you would have noted the very many false starts were Russia was announcing the deal was all but done, while China seemed to show little interest.
It was blatantly obvious the Russians were far more interested in the deal than the Chinese.
Most of the improvements on the Su35 over legacy flankers are changed to internal structures, so would be hard if not impossible to retrofit to existing flankers.
If China really wanted Su35s to study to help improve their J11 family, it makes no sense for China to drag its feet for so long until now, when J11D and J16 development are all but finished, so it would be hard if not impossible to apply anything learnt from the Su35 to their designs without adding massive delays to their programmes, not to mention the cost escalation.
Same with the other often mentioned possibility of China wanting the engines for the J20. Way too late in the day for that to be a good idea now.
2. use as an adversary aircraft in mock fights (best simulating MKI in my opinion).
I'm not sure even the US military would be so profligate as to spend that kind of money on aggressors.
3. use over South China Sea and/or East China Sea so they can freely operate without fear of giving
away sensitive electronic information on PLAAF avionics.
That is the most likely of the 3 possibilities, and I myself had suggested that also in the past.
It will certainly be a factor, but I cannot see that by itself as being decisive enough to green light the deal.
If anything, China's new SCS islands makes the Su35 far less relevant, as its range is no longer needed for SCS operations.
The rumoured location of the Su35 also doesn't fit.
It is actually in the East China Sea, against the Japanese that China has the most need of fighters that can freely use their radars in war mode to respond to Japanese fighters locking radar on Chinese fighters.
There are no such extreme provocations in the SCS, and it makes no sense for China to escalate tensions in the region so drastically when it appears to be doing all it can to limit the possible excuses for the likes of the US to insert itself into the region and its local disputes.
In my view, the thing that finally pushed the deal through was American action in the SCS.
This deal was a hedge against the risk of a real military clash.
Whether the Su35 is better or worse than the latest J11s are irrelevant if China's primary reason for the deal was to quickly boost its fighter inventory to prepare for possible conflict.
That fits with both the rumours of Chinese demands to intergrate Chinese data links, weapons and maybe even sensors onto the Su35 (also maybe why they settled for downgraded version if they were planning on stripping out the radars and other key avionics and replacing them with Chinese systems anyways) and also the secrecy from China surrounding this deal. That is in stark contrast to the openness with other projects like th J20 in recent years.
The Su35 is still a flanker, so Chinese pilots should be able to convert to the type very quickly.
24 planes is a whole year's worth of production, and is nothing to sneeze at.
Given Trumps words and actions since taking office, I would not be surprised if China buys more Su35s in the near future.