Here's when an F-15 is better than an F-22 or an F-35
The F-15 is a traditional air superiority fighter of the fourth generation. It's big, fast, agile, and carriers lots of weapons under the wing where everyone can see them. For that reason, it's terrible at stealth, but the other side of the coin is that it's perfect for intercepting enemy aircraft.
Bronk says that when it comes to interception, a plane would "have to get up right next to the aircraft, fly alongside, show weapons, go on guard frequency, tell them they're being intercepted, that they're on course to violate airspace, and to turn back immediately."
An F-22 or F-35 shouldn't, and in some cases, can't do that.
The major advantage of fifth generation aircraft is their stealth abilities and situational awareness. Even the best aircraft in the world would be lucky to lay eyes on any fifth generation fighter, which means that they can set up and control the engagement entirely on their terms.
But while this paradigm lends itself ideally to fighting and killing, interception is a different beast.
The advantages of the F-22, and particularly the F-35, greatly diminish once planes get within visual range of each other. Also, fifth gens usually carry their munitions inside internal bomb bays, which is great for stealth, but doesn't really strike the same note that starring down an AIM-9 Sidewinder missile on the side of an F-15 would.
Simply put, a fifth gen revealing itself to a legacy fighter would be akin to a hunter laying down his gun before confronting a wild beast.
"Fifth gen fighters are not really necessary for that... other, cheaper interceptors can do the job," said Bronk.
Furthermore, interception happens way more frequently than air-to-air combat. The last time a US Air Force fighter shot down an enemy plane, it was their own wayward drone over Afghanistan in 2009. Meanwhile, interceptions happen all the time, with the Baltics and the South China Sea being particular hot spots.
The fifth-gens, however, make sense for entering contested air space. If the US wanted to enter North Korean, or Iranian air space, it wouldn't just be to show off, and according to Bronk, their stealth and situational awareness would afford them the opportunity to slip in hit their marks, and slip out undetected, unlike an F-15.
In interception situations, it makes no sense to offer up an F-22 or F-35 as a handicapped target to an older legacy plane. F-15s are more than capable of delivering the message themselves, and whoever they intercept will know that the full force of the US Air Force, including fifth-gens, stands behind them.