There seem to be a few ideas contained above that I am not sure are really accurate or valid. For instance, there is a very real question about whether a mixed fleet of ~29 AH-64 Apaches and 22 Tiger ARH's would provide an increase in capability. If (and a big IF, IMO) the Tiger ARH's were to get upgrades to keep it relevant, then it might be able to provide a capability increase. The unfortunate reality though does seem to be that the costs to upgrade the 22 Tiger ARH's is sufficiently high that it is less overall expensive to just replace the type with a different helicopter with the desired capabilities. This is even before the costs, resources and effort which would be required to sustain two different types of recon/gunship helicopters are considered.
If Australia were to go ahead and spend to acquire AH-64 Apaches, and then spend what would be needed to keep the Tiger ARH around, relevant and useful, IMO that same amount of coin could be spent just getting more Apaches and all the needed bits, bobs and personnel necessary to support an increase in the Apache fleet size.
Consider this. In order for the Tiger ARH to be kept in service, training streams are needed to provide pilots, ground crew, and maintainers. The ADF needs to have physical plant/infrastructure in place to operate and maintain the Tiger ARH's from, as well as to hold supplies of spares that Australia maintains, and/or parts which being shipped between Australia and OEM suppliers. On a side note about that, it does seem that one of the issues with the Tiger ARH has been the costs/time associated with shipping parts back to Europe for repair/reconditioning. If Australia were to keep the Tiger ARH in service, such problems would continue to persist and would likely get worse if no new orders get placed to keep production going.
If Australia is going to make the efforts required to have a helicopter recon/gunship capability, IMO it would be far better to dedicate such resources to a platform that can be had and sustained in the numbers needed with the capabilities required.
In some ways, it does seem as though some people want to keep the Tiger ARH in service, simply because Australia already has them in service. In that regard, it seems almost like people would argue that the ASLAV's and M113's currently in Australian service should be retained when the relevant LAND 400 phases provide their replacement platforms, because the ASLAV's and M113's are already owned/operated by Australia.