I will agree that going with an all OPV naval patrol force would be a compromise, but I would disagree that it would be a solution of any sort.
After all, if the RNZN were to find itself without any naval combat forces, NZ would be incapable of escorting vessels and would be forced to rely upon friends/allies.
The entirety of the NZDF's ASuW and ASW capabilities would rest upon the P-8 Poseidons and possibly any armed naval helicopters, if there were to get acquired. Also, if the RNZN is left without surface combatants, the utility and value of naval helicopters like the MH-60R 'Romeo' Seahawk or NFH90 is significantly if not entirely diminished, which in turn would mean it would make little sense spending so much coin on a platform where 80% of the capabilities cannot be utilized.
From my POV, GBAD and land-based AShM would be an expensive way to waste significant coin, coin that the NZDF has never had enough of over the last 30+ years. Such capabilities would really only possibly become useful if NZ were to get directly threatened or attacked following an outbreak of hostilities. Even if NZ were to suddenly become a target though, these land-based systems would also need to be positioned somewhere within NZ where both the sensors could detect and track potential targets, and the launched missiles could then hit those targets. A unit based in Auckland would be of little use in defending Wellington, never mind anywhere on South Island. By the same token, if there was a unit based in/near Christchurch, then Auckland would not be covered, never mind ports and harbours further north like Whangaroa or Whangarei.
Also, any land-based units would be worth SFA in keeping the SLOC to NZ open and/or protecting essential merchant marine shipping traffic like tankers. All a hostile force would need to do would be to intercept or interdict NZ-bound tankers say 300 n miles away from NZ ports and NZ would be effectively unable to do anything about it. All the while Kiwi petroleum supplies would begin to dwindle. If enough such tankers get lost along the SLOC leading to NZ, then NZ could effectively run out of power and have little ability to do anything.