The f15 is going to be shoot down 60km from target by s400s, the f35 can get close enough to drop an harm on them at a speed they cannot intercept, so there's t

The f15 is going to be shoot down 60km from target by s400s, the f35 can get close enough to drop an harm on them at a speed they cannot intercept, so there's that. Also the f15 has speed or range, and if you need one it reduces the other.