His acute modesty means there are concerns that extra demands, such as media, sponsor and strategy-building commitments, might dilute his abilities. A school of thought remains that he should be chaperoned into the role later.
That is why it makes sense for him to skipper New Zealand at the World T20 in India to get his bearings and work out whether he wants to continue.
A youthful disposition should not distract from the fact the 25-year-old is a relative cricketing sage to others his age. He has the potential to be a world-class do-as-I-do leader, but should not be shoved into it. He will face immediate scrutiny, especially with test tours to India and South Africa planned, where New Zealand have never won a series.
The alternatives are Tim Southee, BJ Watling or Tom Latham, but Williamson is the logical preference. Just don't expect McCullum-esque funky fields.
Williamson has led New Zealand in limited-overs matches in the UAE, South Africa, West Indies, Zimbabwe and one at home against Sri Lanka last summer.
In 15 ODIs, he has a seven-win, eight-loss record. In seven T20Is, he has a three-win, four-loss record, often with diluted-strength teams.
Those results include a 3-2 ODI series victory away against Pakistan last December, when Williamson top-scored with 97 in the decider. He scored 346 runs at an average of 86.50 and strike rate of 90 across the series.
Williamson looked flustered on occasions juggling the bowling changes, field settings and over rates, but that's part of the territory adjusting to any new job, especially in tight matches.
Experience should not be a problem. He has now played 46 tests, 85 ODIs and 25 T20Is.
By comparison, Stephen Fleming assumed the reins aged 23, having played 24 tests and 55 ODIs. Richie McCaw took over the permanent captaincy of the All Blacks aged 25, having played 36 tests (he stood in for Tana Umaga in two prior matches).
Like McCaw, Williamson is a guaranteed selection. Such a luxury should not be taken for granted, and Williamson's passive nature should not be misunderstood because it camouflages a steel core. Anyone who has watched him practise would attest to his discipline.
Much has been made of the successful team culture under McCullum and coach Mike Hesson but Williamson's devotion to a team ethos has been at its centre.
If he brings those components to the captaincy, and teammates - who already respect him as a player and person - feel compelled to follow, New Zealand should extend their legacy.
Williamson is more of an introvert to McCullum's extrovert, and it will take years to accumulate the incumbent's chutzpah, but the World T20 presents a prime opportunity for the heir to set a benchmark.