The thing is you don't know whether the method you're calling can return null. It's invisible to the type system. You can try to track the null-validity of ever

The thing is you don't know whether the method you're calling can return null. It's invisible to the type system. You can try to track the null-validity of every value with documentation, but this is tedious, error-prone, and the exact same problem types exist to solve accurately at compile time. If you try to add defensive null handling everywhere it makes for absurdly verbose code, and it's pretty dubious since you don't really know what the semantic meaning of the phantom nulls you're pre-emptively defending against are. Everyone everywhere always being more careful is the bandaid solution, if you lift the extra non-optional sum type into the type system you solve the problem permanently.