As JV has touched on earlier, the major problem is that Kirk really wasn't a well man. Over the years, I have come across a number of anecdotal accounts as to the possible real extent of Kirk's ill health, often from seemingly unlikely sources. The most recent one I have come across: the autobiography of the now late Captain Ian Bradley RNZN, "Don't Rock The Boat". I don't have my copy at hand, but I recall a passage where Bradley encounters a RNZAF Hercules that Kirk has been using, and the array of medical equipment being carried by the Herc sees Bradley believe that the full extent of Kirk's health issues is being concealed from the general public.
A former Muldoon Government cabinet minister I worked with for a number of years (a man who if anything was sympathetic to Kirk's ambitions) is firmly of the opinion that Kirk died believing that his government had failed/would fail.
Kirk was a man in a hurry, with ill health and two election defeats behind him already (hard to imagine any party allowing a leader that many bites of the cherry today). To ride out the impact of the oil shocks and British EEC entry on the NZ economy he would need to moderate some of his immediate ambitions, and I am hard pressed to seeing him doing that.
I quite agree that a well Kirk may well have seen the '75 election being a closer run affair. However, a more likely possibility would have been a contest between a reduced, tired Kirk and Muldoon at the peak of his powers. In this scenario the "what if" mystique aspect of the Kirk Government to an extent remains, but is diminished by his personal involvement in the 1975 defeat.
Regardless, the lesson duly noted by Labour's young up and comers: the vital need to secure a second term.