How many recorded kills has the Su-57 performed in air combat? I believe the number is one. I agree the S- 400 will definitely be a problem. However, a lot of this is theoretical. In actual combat, that is really the only real world example we have at the moment of a direct conflict between Russian and US forces and the Russians were whipped something bad.
Also, it's telling that most military leaders in the US do not view Russia as it's direct competition. They are more concerned about China. Since the fall of the Soviet union, the Russian military has had terrible organizational structure and their maintenance of their equipment has been poor. This is what happens when your government is ruled by a single autocrat and a bunch of corrupt oligarchs who pilfer industries to line their pockets after your previous government collapsed. Money and resources that would be going towards a common defense get dispersed inefficiently because people are constantly taking a cut for themselves. We have corruption but in Russia it has led to real deterioration of their forces. Funny enough, China has some of the same issues. Top down organizational structures and lack of accountability for one level down individuals (oligarchs in Russia, regional governors in China) encourages institutional rot. This includes rotting of your military.