I'm concerned for the J-31 rather than J-20. The J-20 is never going to be " thrown away" . It is a neat aircraft that'll ( or must) see into that China is "blessed" with a stealth fighter with 180 KN thrust engines. It is it's primary duty. China acquiring SU57 would be to deny India the chance for acquiring it or dent the confidence of India in the aircraft ( "if my enemy has the same aircraft, then i'm worried that he'll know my weaknesses" ). But J-31 is a concern. I've read that it isn't structurally or aerodynamically a great aircraft, relative to the flankers. I hope they iron out its defects so that they'll have a Hi-Lo mix of stealth fighters. Recently' Janes have reported on a J-18 Snowy Owl STOVL. Then there is the JH-XX and the H-20 etc. Too many programs, too high a chance that one or too would turn out to be cursed to live as museum pieces that present itself as fancy chinese X-planes.