There were the TVC engines and later we found out about the airframe enhancements. And even if the Su-57E had a few tricks up its sleeve that somehow went unnoticed by keen military observers, are they really worth the money and headache that goes into integrating them?
That depends on how much money it would be.
A better question at this stage is to ask whether we can be assured that in the medium to long term future that it would always never be worth the money for the PLA to acquire a small number of Su-57s. I'm not convinced we can rule it out.
I'll reply in point form:
1. Exposure: I do not see how this justifies spending billions of dollars and complicating integration/logistical issues. What other air force hostile to China will operate these jets? If you're arguing that the PLAAF could learn a few things from the Su-57's design, then the PLA firstly would need to be convinced that the Su-57 offers a distinct advantage or two over the J-20 and that those advantages are critical enough to warrant the purchase.
2. DACT: This bridges to the above point. What is the point of DACT with a Su-57 when you know that none of your immediate adversaries will operate this airframe?
3. Peacetime use: That is why the J-20 carries Luneberg lens and shuts off its radar in peacetime. You don't need a separate 5th generation platform to do that.
If we're to go by the logic of "if X can potentially do Y, then perhaps we need to by X", then China should place an order for every Russian platform out there. Money doesn't grow on trees and spending that money requires serious justification.
1. Similar arguments could've been made about Su-35s, and those arguments were made. I made a few of them myself in the past.
2. I think the idea of DACT isn't necessarily to have air combat training against an aircraft that opponents will operate, but rather merely to have air combat training against a "dissimilar" type. Of course, there is also the good chance that India will end up buying Su-57s in the longer term too.
3. That's assuming J-20s deployed outside of China's borders during peacetime wont' have a reason to emit actively, or that the PLA are comfortable with allowing their IR signatures get revealed, or that there are no ways for the RCS of J-20 to be otherwise extrapolated despite J-20s luneberg lens.
Just for the record, at this stage I don't think China is going to buy the Su-57, but I do think that we can come up with a few serviceable reasons for why they might consider it.
For the longest time I came up with similar reasons for why I think China wouldn't buy Su-35s, but they ended up doing so as well. So I think it would be an overextension for us to say that China will never buy Su-57s in the future either.
A purchase of Su-57s would very much be a luxurious afterthought for the PLA and not contribute heavily to the overall 5th generation combat fleet of the PLA and likely not be part of the PLA's system of systems.
