I didn't have this perspective prior to reflection on
@SkidAce's examples, but here is what I'm thinking. Per PHB 174 and DMG 237, in 5e DM calls for a check only when the outcome is
uncertain.
Say as DM, I set a DC that as it turns out, results in
certainty? By the 5e rules, were it a certainty I shouldn't have called for a check. The outcome of calling for the check is paradoxical (the check shouldn't have been called for.)
Under the proposed 1e rules, that paradox never arises: it is always true that when DM calls for a check, success and failure are both possible. Assuming something like DMG 237 is retained (not guaranteed) then DM needs only to think about the fiction.