If
Shaun Johnson was earmarked as a future captain as a 20 year old then why not chuck him in then? At what point do you say a time frame is irrelevant in terms of development for a role? I would say an extra year or two for a half that is still learning his trade at
Shaun Johnson's age is vital to his development as a player, without landing him with all the extra shit that comes with being a captain. Some players relish the extra responsiblity, some players prefer to be in a leadership role without taking on all those little extras expected of a club leader.
Shaun Johnson may well be ready to lead a side if he is required, but it doesn't necessarily make him the best man for the job at this point in time. If you have a more senior player who also feels ready to lead the side, who wants the job and feels he can really make a difference - then why stand in his way because there is another, younger heir apparent?
Shaun Johnson may turn around in a couple of years, hopefully as a more well rounded player with some finals success under his belt and say 'you know what, I've played under a couple of different captains now (as opposed to just one), I've learnt some do's and dont's, I feel more comfortable with my role in this side both on and off the field, I'm not coming off a long injury lay off and now I feel I am ready to properly captain this club.' That's obviously an ideal scenario, but I'm not of the opinion that a year or two isn't worth anything in preparing someone for a job like captain.
Also, if the players don't like his style, because presumably it's a tad more unforgiving/harsh than what they're used to, then lord help us. Changing captains will simply be re-arranging deck chairs on the Titanic, if this culture (or lack thereof) persists.