Victory by stealth for those who support the accepted Maori spelling seems the inevitable outcome. It may well come against the wishes of the majority of the citizens of Wanganui, but people adjust to change. Nobody now complains that women have the vote, and gay marriage is gradually becoming more acceptable.
The argument over the spelling has aroused strong feelings in the past, but a lot of people have got over it and moved on. The debate doesn't have the same capacity for antagonism.
Of course, officially the options of "Wanganui" and "Whanganui" are both recognised, and we are all free to make our own choice.
At the end of the day - just like "Petre" - it's only a name.