Eight of the 10 full International Cricket Council members host T20 leagues, covering less than eight months of the year.
McCullum is an extreme example, but New Zealand top 20 contracted players in the mid-rankings might consider their alternatives.
It's nothing new for players outside Australia, England and India to exercise such options.
West Indian Chris Gayle stopped playing tests in September 2014 and has not played an ODI since the 2015 World Cup after financial disputes with his board. He has played T20s in the domestic leagues of seven countries.
Fellow Caribbean cricketers such as Sunil Narine, Andre Russell and Kieron Pollard have followed his lead.
New Zealand Cricket, aside from recently refusing Ross Taylor a no-objection certificate with the Melbourne Renegades on the basis of travel protocols, tend to be flexible with player requests.
Such pragmatism includes an IPL window of at least five weeks, and the right to contract in the England, Australian and Caribbean leagues outside international tours.
However, NZC need to be vigilant with the burgeoning nature of T20 leagues.
The decision by Colin Munro, one of the game's pre-eminent hitters, to sign with the Sydney Sixers is an example. It would be understandable if he pursued T20 endeavours ahead of international honours.
His scenario prompted McCullum to issue a warning on Newstalk ZB: "We can put our heads in the sand and say everyone should want to represent their country, and I think first and foremost people do, but there's an opportunity cost as well.
"You've got to do the maths. There will certainly be guys in New Zealand who will be thinking the T20 circuit is for them.
"The danger is that you have to earn the right as well ... and that's where New Zealand Cricket has been excellent for us, they've understood it and given us opportunities."
NZC chief executive David White felt the situation was in hand for now. He told Newstalk ZB's Tony Veitch "the flexibility works well and I can't see that escalating in the near future".
Yet no meaningful test championship looms as an antidote to T20 leagues gathering momentum.
The ICC board meet in Dubai next month to discuss the issue, but have dithered over what format a championship should take.
White said the latest proposal involved two pools rather than two tiers and agreed "we need a structured competition to give it context".
Last year, ICC chief executive Dave Richardson said the earliest the concept could be implemented was 2019 when broadcasting deals are renewed. That's despite the Federation of International Cricketers' Associations claiming 72 per cent of players were in favour.
Eventually increased player migration from the test arena to the T20 merry-go-round could force a decision. The boards might rue their hesitation.
