Money, money, money - personnel, personnel, personnel - there is hardly enough Defence dollars/trained personnel around to keep older and usually more expensive per flying hour machinery in service to support new equipment until the new equipment is "in service" let alone to operate these aging assets in parallel once the new machinery is "in service". For example, many people would love to keep the SK50/UH1H in service but it is not practical/cost effective in a Defence Force the size of Australia's. But now, on the other hand, if one Service operated all the aviation assets.....
GAGS
E86