Key could perhaps coax an extra two years out of McCullum, through to the home Ashes in 2027 and the next 50-over World Cup that immediately follows in South Africa, Zimbabwe and Namibia.
Clearly, there are practical hurdles to overcome when it comes to doubling the workload of a man who lives on the other side of the world.
But these issues would not be insurmountable with some careful managing of his schedule and load-sharing among England’s assistant coaches, with whom McCullum already has an excellent working relationship.
Take this winter, for example. Before Christmas, England have Test tours in Pakistan in October and a favourable trip from McCullum’s point of view to New Zealand in December. Sandwiched in between is a limited-overs tour to the Caribbean in November, which interim coach Marcus Trescothick may lead anyway if England don’t have a permanent plan in place by then.
In 2025, McCullum isn’t due to work for England for almost five whole months until the one-off Test against Zimbabwe in May. That is an opportunity to take charge for the Champions Trophy in Pakistan in the spring and the warm-up tour of India that precedes it.
As time moves on, obvious series and tours to give McCullum a break would arise. Not only that, giving chances to lead to the likes of Trescothick, Paul Collingwood and Andrew Flintoff can only be a good thing for an English game that has an appalling record in turning its very best players into top coaches.
At the moment, England are going through the process of finding a new white-ball supremo. There is no indication McCullum is being considered or would want it. If he were to assume responsibility for both teams, there is the risk of burning out one of English cricket’s most prized assets, even if the man himself is unflinchingly unflappable.
There are others who may want the job, too. Trescothick has not ruled himself out of the running and Sri Lanka batting legend Kumar Sangakkara called it “an exciting prospect”. Andy Flower, an Ashes-winning coach who was once the Test man in a job-split scenario, is one of the most sought-after leaders on the franchise circuit.
As it stands, there is a question for Key and England. They may already be in possession of the answer.