It is more complex than that.
Fact 1 - if NZ had better batsmen solely, Hadlee's wpm potential increases by virtue of his bowling increasing in second innings. His second innings average is lower than his first, so this will likely reduce too.
Probable Argument - if NZ had better support bowlers, Hadlee's wpm may stay the same or decrease pending how many there were, as more games go into the second innings, but if it decreases (say 3 great support bowlers) his bowling average would likely reduce as bowling more balls to non set batsmen.
This is all based on known truths, you need runs on the board to make the opposition bat twice, and batsmen are most likely to get out when new.
Now you can take Hadlee's first and second innings split and compare it to McGrath and Lillee who had Punter and Waugh, Hayden, and Chappel and Chappel respectively - and it will reveal itself to you.
Or you can avoid the fact that their first and second innings splits are so vastly different. Over to you.
I wouldn't be surprised if Marshall's Windies and McGrath's Australia averaged 18 to 19 wickets per game and Hadlee's NZ about 12 maybe 13 at best. But to say NZ "lacked competition for wickets" is erroneous, cos there were wickets there to be taken, they just weren't. There was no competition as the wickets were so often not taken at all. That is why SK Warne's argument is fallacious.