I agree with you re: NULL being a useful thing. I personally use nullable floats in an internal company program to denote unknown values. However, the "billion-dollar mistake" everyone brings up with it has to do with NULL allowance being implicit. In languages like C/C++, Java, C#[a] (and more), any pointer could be NULL and the only way to know is to do a NULL check. In SQL (which we're talking about here), one must explicitly call out `NOT NULL` in the column's definition.[b] Rust (and other FP languages) gets a point here by having "optional" types one must use to have a NULL-like system.

[a]: C# is fixing this with "nullable reference types", but as long as it's still opt-in, it's not perfect. I can still forcibly pass a NULL to a function (defined to not take a null value) with the null-forgiving operator: `null!`. This means library code still needs `ArgumentNullException.ThrowIfNull(arg)` guards everywhere, just in case the caller is stupid. One could argue this is the caller shooting themselves in the foot like `Option.unwrap_unchecked` in Rust, but "good practice" in C# says one should have guard checks just in case due to NRT not being present for over a decade of the language's lifetime.

[b]: Which is kind of stupid, IMO. Why should `my_column BOOL` be able to be null in the first place? Nullable pointers I can understand, but implicitly nullable everything is a horrible idea.