I think it would be nice if `NOT NULL` was set by default on columns. However there are a lot of legitimate use cases that can't be (practically) solved by restructuring.

Data can be incomplete. Maybe only because it is not (yet) known. If NULL values were impossible it would create the need for one additional table with a foreign key relationship for every attribute that can be independently NULL. Sometimes this pattern is a good idea, but I don't think I can be convinced it should be the only possible solution.