The U.S. may not have enough pilots to win a long war against a peer competitor. It takes a long time to train pilots, and they don’t stick around because flying commercially pays more. The pilots we do have don’t get enough flying hours. The U.S. Air Force has similarities to Japan and Germany’s air forces at the beginning in WWII. Germany and Japan did not have enough pilots or a robust enough pilot pipeline to maintain skilled pilots throughout the war.
Russia’s S-400 anti-air missile batteries are good, but without being integrated into larger defense systems, are vulnerable to air power. U.S. air power out-ranges these missiles, and low flying aircraft can’t be seen because the S-400 by itself can’t see over the horizon.