Problem is you end up other places with the assumption thar it's never null. So in the future when you actually set it to null somewhere it will blow up.

People that assume that a (nullable) value is never null because the sample at hand does not contain a null value _will_ learn their lesson the hard way, the one you describe.

But even non-nullable does not always resist to time, I'd argue that use cases where the field _has_ to be null eventually emerges and somehow have to be mitigated. There is no easy solution to safely workaround that without either tons work that duplicates lots of things or taking risks by adapting the code base.