Only those who either ignore or are unaware of the cost of maintaining a maintenance capability for a single type that no longer has any commonality with anything else in the military logistics chain would suggest that keeping a "orphan" aircraft type could be cheaper than utilizing an existing type for an extra mission-set.
Start with training the maintenance personnel (you will need more for a separate type than for a few more of an in-service type), then add in the costs of upkeep on maintenance equipment particular to that type, the costs of parts (most of which are no longer commercially made) for that equipment (resulting in having to buy new equipment to do the job), the costs of parts for the helos (some of which are no longer made, thus requiring a modernization program to replace those systems with ones that are still made/supported), and the other costs associated with a singular aircraft type.
Compare that to the costs for a type that is already in service that you are simply using for a different mission-set instead of what you would be using them for.