Just to stir the pot: if a 189 is down then I assume the S92 stands in short term (assuming the one spare 189 is allocated elsewhere/unserviceable) and equivalent/better coverage is available.
What if the S92 is down and no spare 92 is available? Aren't the two types of different ranges and capability, so the second aircraft at the affected location may be unable to achieve the range required for the job?
All the SAR machines being phased out are of a like type and like capability: to infer that numbers alone are the criteria to be considered seems a blinkered view. A 189 doesn't seem to be a subsitute for either an S92, nor for a SAR Sea King.