Not quite missing the point. I'm sure there would be a product line up, similar to the Impreza. (Base, WRX, STI) For the 2009 era, you had another spot in there when they decided the 08 WRX's were not what the people wanted and sold them off as the 2.5 GT.
So you take the already created platform, add the upgraded package to it, sway bars to deal with the additional weight up front since you mentioned that already, and you have a very diverse product that reached that many more consumers. It's a world car, so it only makes sense to push the development of it into the beastly STI package sooner of later.
Subaru has done well, and gained a huge following with this business model. How many of us bought STI take off parts to use for upgrades on our less performance spec models? I know A LOT of people that do this. Most of us. It could/would be the same way with the BRZ. Only this time, it's not a crappy base model Impreza L that you swap a complete drivetrain into, it's an already sporty and fun driving car.