Chance and the existing future
Unpredictability, indeterminism, and an unwritten future are separate claims. Quantum uncertainty does not settle the ontology of time by itself.
An event can be definite in a complete history without being uniquely entailed by its antecedents. This is the essential distinction between an existing future and a deterministic future. Eternalism says which temporal events exist. Determinism says whether a physical state and the laws permit only one subsequent history.
Suppose a theory assigns several possible outcomes to a physical process, with no earlier condition selecting one of them. A complete history could still contain a definite outcome of that process. Describing its result would not show that the result had been causally necessitated. The distinction is conceptual; establishing which theory describes our universe requires physical evidence.
There is an equally important limit on the argument. A result’s becoming definite after an event does not establish that it existed before the event became present. A defender of an open future accepts definite past outcomes. They dispute the existence of future outcomes. Appealing to the settled past therefore cannot prove eternalism against them.
Quantum unpredictability consequently provides no quick verdict in either direction. It does not by itself establish an ontologically open future, and it does not become mere ignorance just because one adopts an eternalist picture. One must first explain what the quantum state represents, what produces or constitutes an outcome, and how that account relates to temporal existence. The philosophical discussion of time and physics treats those connections as questions requiring further argument.
This also limits an appeal to randomness as a source of freedom. The failure of prior conditions to necessitate an event does not give a person control over it. A randomly occurring decision could remain just as unavailable to the agent’s authorship as a determined one. An account of free will needs an account of control, rather than a gap where prediction fails.
The case for eternalism should therefore rest on its account of temporal structure, including its relationship to relativity. It becomes weaker when it claims that every rival conception of quantum probability has already been reduced to ignorance. A complete reality and a causally necessitated reality may coincide; treating them as synonyms conceals the question that matters.