The first ever trophy for them in an ICC global tournament. Test cricket is regarded as the toughest format and New Zealand should be proud of the fact that they managed to overcome the challenge from the Big 3 teams. And the three South African imports played a large role in this victory (BJ Watling, Devon Conway and Neil Wagner), so they should thank the South African quota system as well.
What if there were two days lost in rain, should they extend the match to 7 days ?
I disagree. Almost three days of play was washed out (including entire days play on 1st and 4th days) and despite that New Zealand managed to win the test match, and thereby the first ever World Test Championship.