What are you talking about?, no one's going to take your 360 away from you.

then why make a $150 peripheral if it is just doing the 360s job?

You're correct. You can keep your 360 if you want, the mini just adds the benefit that you won't need to swap consoles everytime you want to play a 360/ Durango game. The Mini will be made specifically to work with the durango, just like how the kinect was made to work with the 360 slim, while older 360s needed an update.

So paying an extra $150 for something that should just be already included in the console is considered good now?

adding backwards compatibility is harder and more expensive than it looks. It's this, or spend an extra 100 dollars on the Durango.