If my understanding of the history of it is correct, fifty years ago would have been before the modern approach to probability spaces developed by e.g. Mackey really caught on.

To distinguish "surely" from "almost surely" requires more than the measure algebra, the most reasonable way to do it is to say that "surely" refers to the support of the measure and "never" refers to anything not in the support of the measure. The problem here is that the support is a topological artifact of the particular compact model (also called point realization in Mackey's terms) and not a measurable object. In particular, what one would consider "sure" to happen is not an invariant of measure-isomorphism. So when we use a probability space to model something, we cannot turn around and use words like "impossible" without modifying our model to include something beyond just a probability space.

Certainly nowadays the probabilists simply don't use "surely" or "never" without an "almost" in front of them, and it's for exactly these reasons.