The mistake is being nullable/optional by "default", that is with the least amount of effort for programmers using such a language. Or worse only ever nullable

The mistake is being nullable/optional by "default", that is with the least amount of effort for programmers using such a language. Or worse only ever nullable (like Java is except for its built-in scalars I think?). There is obviously a need about having optional things, but this is not the common case, so this should not be the default and even less the only solution. And it should enforce handling the absent case. "null" is a shortcut for talking about solution which does nothing of that (and is even UB in case of mistake in some languages). Billion Dollar Mistake is generously low; probably the cost is already Multi-Billion Dollar, and counting.