That's theoretic talk. Try doing it in practice and still get things done / be able to maintain the code / see the forest for the trees.
But I have a feeling that we are lacking a bit of context here. Some people seem to focus on web-application style of programming (understandably) where you have lots of trust issues. Whenever data is carried across trust boundaries it needs to be checked (this applies to integrity in general, of which null safety is just a small part).
(On the other hand, deserialization is not about validation of function arguments. Deserializers should assert integrity on the spot before calling into deeper nested functions).
