Cost of J-20: 120 million $
Cost of J-31: 70 million $
Cost of Su-57: 40-50 million $
Based on that, Su-57 doesn’t need any additional merit to already be considered a serious contender.
Besides, the plane will proliferate, so having some would be useful for training purposes.
The main flaw of the Su-57 is that it has not made up it’s mind to use stealth or maneuverability. However, that flaw can easily be mitigated by pairing it up with J-20s, the same way J-20s would be paired by up with 4.5gen otherwise.
Finally, like the Flanker design, it would have high upgrade potential. While the J-31 is stuck with smaller WS-13 engines, which are not revolutionary, an upgraded Su-57 can benefit from J-20’s engines. A larger design also means more room for avionics and eventual sensor upgrades.
Even if a comprehensive contract is not signed, China still has good reason to get a handful for training purposes, like it was done with Su-35.