I guess you are either saying (A) to treat every return type as possibly null or (B) to use your judgment when doing null checks of return types.
In case of (A): Do you want to write null-checks for methods like String.toUpperCase() or for user.getLastName()? That would lead to a lot of bloated, dead code. Also do you want to write error handling for all these cases you know can never happen?
In case of (B): NullPointerExceptions are most likely when the user of an API did not expect he might get a null value. For instance user.getDepartment -> null. "What? An employee does not have to have a department?!" In this case you force users of an API to guess if they might reive a null return value. And they might guess wrong, which either leads to dead code or NPEs. Using Optional as a return type makes the difference explicit.