> This feels like a footgun for when a developer overrides/implements some kind of callback and returns null where the overridden method specifies non-null return values.

I've caught "null!" in C# code reviews a few times: My response is usually "You can't fix your code by yelling at it."

The reality is that C#'s approach also has some nasty footguns that require understanding the nuances of the "required" keyword, and when values are assigned.