Trouble with the kiwi mentality is that we don't think we are 10ft tall and bullet proof - we know that we are. Anything bad is always going to happen to somebody else and with our nearest neighbours being the West Island, the pollies and general public think that all bad things start north of the Top End and stay there. The RNZN said recently that 99% of our trade is seaborne and Mr C, I understand what you are saying in your first post. I agree with you on the political and public perception dimensions. I'd actually forgotten about Resolution and the non replacement of that hull.
Resolution was partly used as a hydrographic vessel and from what I understand the multibeam echo sounders and motion compensators off Resolution were supposed to be transferred to the OPVs. Whether that is a permanent fitting, or as required I don't know, but permanent would be better because if it's temporary would really be a pain, as set up each time can be complicated. Given that, as far as I am aware, the RNZN no longer has the LINZ hydrographic contract and they have cut back on their scientific work (budget cuts to Defence Science), the only acceptable argument I can now see for an extra OPV or two is the fisheries patrol of the EEZ, Pacific Islands, Sub Antarctic Islands and Antarctic waters we claim.
I think we might be getting "hung up" on the ice issue with the MPSC. I may be wrong, but I would think that the reason for the ice strengthening is so they can use the vessel to carry diesel fuel and aviation kerosene, to McMurdo during the summer months. In my view using a 20,000+ tonne AOR /amphib support vessel for fisheries protection is an expensive and inefficient option, if that was all it was tasked to do for one mission. However if it was returning to DNB from a fuel drop off at McMurdo and were to undertake some fisheries work that is another story.
After Rena I was thinking that maybe instead of the NZG investing in two dedicated oil spill vessels, as suggested by some that modules could be designed to fit on the OPVs and they be used for such work. I think the size of the OPVs are about right and I would have to ask elsewhere about their handling and sea keeping etc., but I think that they could be the basis for a new OPV with better design, because we should have learned some lessons.
If I combine one of Mr Cs and one of Johns comments and ask how do we justify the expense of a sea going ice breaking "salvage" tug? Who is going to operate it? Mr C commented on us not having anything to tow a vessel in trouble in open waters. John suggested an icebreaker; I've just combined the two into something which might be something more viable. Then you have an icebreaker that can take the Endeavour replacement to McMurdo as well as tow / salvage a stranded container vessel, tanker etc., across the open sea. Secondly it saves the USCG sending one of their icebreakers down each summer. The US NSF possibly would be happier as they wouldn't have to bear the cost of the vessels transit from the US Pacific Coast to Hobart and back. Make it a bit larger and yes it could support Antarctic science, but don't forget NIWA have their research vessel Tangora and they could (and do) operate that a fair ways south.
You could possibly sell the MPSC and another OPV with spill kit to the general public on environmental and fisheries protection grounds and maybe even an icebreaker /salvage / Antarctic patrol vessel. The RNZN got a lot of coverage using the Endeavour to take the off loaded fuel from Rena during the salvage, so even the greenies might accept the idea.