The "billion-dollar mistake" was about implicitly nullable values, i.e., allowing a variable with type `T` to also be set to `null`, not null-terminated strings.

Anyway, one argument is that UB is fundamentally useful in languages that are insufficiently type-safe, like C and C++. The "holes" in the specification allow for regions where the compiler can optimize the code in ways you may not expect.

As we have developed more advanced type systems, the utility of undefined behavior has lessened considerably.