The problem with multi use aircraft is that when needed the fire bombing use will be subject to "commercial needs". The only way to guarantee their availability is dedicated airframes and crews.
The RAAF and Qantas to a degree shared a couple of B707s towards the end of the B707s airline life. I don't think either side was all that happy with the conflicts between operations
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The problem of multi types is that standing expenses are high. Different crews, different equipment, different spares etc and different expectations. One of the reasons why air ambulance helicopters are to a degree standardised or were in NSW up to recently
Selection of type would depend on if we are working to a Federal or State operation. Skycranes may be OK in one state, but response time between the West and East would be poor.
Per hour operation and standing costs would be lower on a fixed wing.
Wunwing