In a way it is (there are new features). But the difference is that nightly is for people who accept that something might be broken. In our car analogy MB had to spend one gazillion dollars making sure the features on nightly (S-class) are as stable as you'd expect from the battle tested version. Apple will have to do the same thing. Even though the oled screens and facial recognition is new tech, their $1k-paying customers aren't happy if it doesn't just work perfectly anyway.

Not an exact analogy as an s-class car will be held to even higher standards and "bugs" will not be tolerated or excused as price for cutting edge features. Both are stable releases