Isn't that the classic argument "Real C programmers don't write defaults!" ?

The one that companies have spent billions of dollars fixing, including creating new restrictive languages?

I mean, I get the point of tests, but if your language obviates the need for some tests, it's a win for everyone. And as for the "how much code will I need to change to propagate this null?", the type system will tell you all the places where it might have an impact; once it compiles again, you can be fairly sure that you handled it in every place.