I'd also say people are buying the STI in spite of the EJ25, not 'because of'. In the market place today, the STI is a uniquely 'old new' device; the drivetrain is uniquely good, the 'analogness' of it all is unique in a more and more digital world. Hydraulic steering, the 6 speed, the differentials, all make the car special and 'different'. After having a 2015 WRX w/ the FA for 5 years, I can appreciate what that motor is: It's better. And a slightly amped up version ("Stage 2") with the STI parts (drivetrain) would be a fantastic car; it just doesn't exist today.
So you can either take a crappy 6 speed and 'boring' differential setup (WRX) with a good engine, or an old-and-not-as-good engine and the rest of the parts being better (STI).