The United States Air Force needs aggressor aircraft. There is no geopolitical adversary for the United States quite like Russia and its Soviet-built airplanes. American combat crews need to train against someone, and the best we can get comes in the form of MiG-29 fighters and Sukhoi-27 aircraft.
It doesn't matter that the aircraft are from the 1970s, so is the U.S. Air Force's F-16 fleet. American airmen need targets, and these are the most likely real-world ones.
In 2017, onlookers spotted an F-16 engaged in a life or death dogfight over Nellis Air Force Base, Nev. with a Russian-built Su-27 Flanker aircraft. It's highly unlikely an errant Russian fighter penetrated NORAD and began an attack on a specific base. The only logical explanation was that Nellis has a supply of Russian-built fighters for U.S. airmen to train against. It turns out, that is exactly what happened in the skies over Nevada that day. Make another notch in the win column for Occam's Razor.
The United States Air Force has acquired and maintains a number of Russian and Soviet-built aircraft for airmen to fly against. Where they get the aircraft is anyone's guess, but The National Interest reported it likely gets the most advanced fighters from Ukraine. Other fighters are on loan from private companies who acquired the Russian planes on their own. That's another W for capitalism.
Anything is possible with enough money.
So even if the United States Air Force couldn't afford to own and maintain its own supply of Russian aggressor aircraft, there are apparently a number of civilian contractors who have acquired them and are willing to loan those fighters out to the USAF. Among those come MiG-29s from a company called Air USA, MiG-21s and trainer aircraft from Draken International, and the two aforementioned Sukhoi-27 fighters from Pride International via Ukraine.
Let's see the semi-Communist oligarchs in Moscow pull off acquiring an F-22 Raptor using their shady business dealings. But even if the United States couldn't fight real Russian fighters, American pilots could still get excellent training.
The emperor has new clothes.
If you're not sure what's happening in the photo above, that's an F-16 Fighting Falcon all dressed up as a Sukhoi-57 fifth-generation stealth fighter. While the F-16 may not have stealth and definitely isn't a fifth-gen fighter, it still gives U.S. airmen training on what to look for while engaging a Russian in the skies. The paint job is used by the Russians to make the Su-57 look like a different, smaller aircraft from a distance. Acquiring real enemy aircraft and training under the conditions closest to combat will give American pilots the edge they need.
That is, if they ever need that edge against the Russians.