How does any of that explain why McGrath playing in a better team had a lower wpm than Hadlee?
You want me to translate it from Marshall to McGrath?
The question is not why are great bowlers in great teams more limited in wpm per match than in weaker teams, I think we can see from McGrath at 4.x if there is sufficient batting (and with Gilchrist, Hayden, Langer, Punter, Hussey there more often was than not and they won) it is the competition for wickets limit hits them at the 20 wicket point far more readily [Warne, Gillespie, Flemming] than it does for a great bowler in a weaker team but with good batting like Murali, who ends up with 6 wpm or an even weaker without good batting team like Hadlee at 5 wpm who is being limited not by the 20 wickets taken, but by the team running out runs.
Which factors [actually] limiting wpm potential effecting McGrath, Warne and Marshall - Put McGrath in here , do not effect Murali and Hadlee [the same]. And Murali had more runs to play in the second innings and bowl more hence has a higher wpm than Hadlee.
But I also accept that competition for wickets in reducing wpm also typically reduces bowling averages of the bowlers involved, as there are more chances at bowling to new non set batsmen. So all these dynamics are interrelated.
So even though NZ's Hadlee was a weaker team than the great WI or Aus, he still could have taken more wickets per game if the batting was better. Same for Imran whose 4.6 wpm per match, is not at all like Marshall's! But still better than split than Hadlee's.
Remember there's 20 wickets to be balanced against the runs, competition for wickets kicks in limiting potential when 20 wickets are taken.
Its not just the bowling that matters, its the runs the bowlers have to play with. It is that simple.