Could it be a case that two things can be true at the same time? That is as the AH140 was first off the blocks it had the inside running (which aligns with Rob's sources). But now with the Australia choosing the second generation Mogami it has now forced the NZ Govt to reassess its options and even at this early assessment stage it is clear that the Mogami option provides several attractive factors that you have outlined.
The only question I had then was that of manufacturing offsets (and costs). Whilst an AH140 purchase could provide some NZ industry involvement, it seemed to me a Mogami purchase would likely be a one-way transaction to Japan's advantage (i.e. no local involvement and therefore basically $billions to be sent overseas, a bug bear for small nations like NZ). However this article from the Japan Times is suggesting that the Japan Govt is aware of these issues, so that may not be the case, potentially.
Also of note it quotes the DefMin stating that "
Penk has said the country is looking at all options to modernize its navy and add further new ships".
I think we all here should keep an open mind about the future RNZN fleet composition (and not assume the likes of 1:1 replacements as some people state matter-of-factly here)!
A deal with New Zealand would build on an agreement Tokyo recently signed with Canberra to build the first three of 11 Mogami-class ships planned for Australia.
www.japantimes.co.jp