Maybe Transcript is unnecessary, as it only wrapped in OperationResult? Instead the Operation could include repeated OperationResult which could have ResultType instead of TranscriptType. This would make the result tree shallower and thus easier to understand.

I added the OperationResult as it is expected that there will be result types that are sufficiently different from Transcript in the near future, so they can be added with only additive changes.

that would be possible also by extending the OperationResult itself, it could be just

message OperationResult {
  enum ResultType {
    ...
    RESULT_TYPE_TRANSCRIPT_LEXICAL = 1;
    ..
  }
  ResultType type = 1;
  string text = 2;
  repeated Token tokens = 3;
  // future additions here
  // repeated Event events = 4;
}

This would collect the result alternatives in the same type, instead of an additional intermediate container type.