If China’s Stealth Fighter Is So Good, Why Is Beijing Still Buying Russian Warplanes?
War Is Boring

The function is different, the SU35 is for totally different types of missions than the J-20, the J-20 is much larger with a bigger weapons bay for a reason.

The J-20 is, as has been said before, “An F-111 sized aircraft for the F-111’s original mission”. ie It is intended as a medium bomber “Strike Fighter” and for intercepting huge, unmaneurvering, slow targets over the ocean ‘a Naval Interceptor’ /flying missile truck with a big radar for long range intercepts.

The US Navy was supposed to use their Version of the F-111 to defend carrier task forces from Soviet bombers. Shooting down AWACS, JStars, Tanker aircraft etc is considered more important than going after combat aircraft as these are ‘force multipliers’. It is also probably a lot safer and easier, since they cannot do their jobs without giving their positions away to everyone for hundreds of miles. That has been Russian doctrine for 30 years and the Chinese got at least some of their doctrine from them.

As for the medium bomber, well there are very few places in the western Pacific that for example a USAF force could be based out of and hit targets inside China. the J-20 would have the reach to hit anything in the ‘Inner Island Chain’ and with tanking Guam probably. The J-20 could follow up IRBM attacks on Kadena etc to finish off anything left and stop repairs. The Chinese have short and intermediate missile regiments assigned to that work anyway.