F4 designers at McDonnell Douglas thought the same, but soon realized they still can't ditch the guns. China hasn't fought a major war since the Sino-Vietnamese war in 1979 and even then the airforce didn't see any combat actions. There are no precedence for China to rely on. No one knows if guns on stealth jets are necessary, but Lockheed still put guns on both F22 and F35.Dogfights don't occur like they used to anymore over Vietnam. Modern fourth-generation fighters have engaged other fourth-generation (and older) fighters with missiles, almost completely. Just look at the examples in Iraq, Yugoslavia, Ethiopian-Eritrean War, etc.
And a stealth fighter should not be getting into engagements that require a gun anyway. If it ever gets to that case, it would make sense for the J-20 to retire to the back and allow accompanying fourth-generation fighters (J-10/11/16) in the formation to engage. The guns are with them.
Time will tell.