You have to keep in mind that when the Su-35 was inducted the J-16, let alone J-20, was ready for induction. If neither panned out PLAAF can’t just sit around doing nothing about it. The situation is very different now to say the least.
And even more we all know, the purchase was more a political support towards Russia than a true operational need, even if it was a chance to get an insight into Russia's latest Flanker variant and to gain experience in using a TVC engine in service.
Any such ideas, China could or even would again purchase a Russian fighter and be it only for trial or as an aggressor is plain ridiculous and a desperate attempt to create or keep the impression as if Russia and the Su-57 are still in the same league. This train has left the station since long and the fact that they are only showing a T-50 - in fact IMO pure embarrassment - and not even a real Su-57 or even the prototype with AL-51F makes it quite clear that Russia has finally lost touch.