I disagree on your view that Brendon needs to hunt numbers, I think that his raison d’etre is winning, which has lead to him moving around the order because he thinks it will help the team to win. His 302 has cured how he will be perceived as a batsman (Greatbatch is remembered for the innings at perth not as a guy that only averaged 30) and I don’t think he is capable of the numbers that you suggest given how average his away form is. His current average is almost identical to where it was at when Vettori gave up the captaincy. He will still finish with Most runs (Right Hander), most fifties (Right Hander) and most consecutive games from debut (Internationally).
If you look at the current international calendar post world cup he is only scheduled for 4 weeks off till the end of the next NZ summer, he could potentially drop Zim series and ODIs but it is still very hectic.
BMac has already stated he wants to tick a few more statistic milestones off the list, he mentioned he'd come close to retirement due to his debilitating back problems after the English series but that with time off, his back was manageable again and he didn't see his retirement coming in the next few years.
After the world cup he will be 33, though young by many international standards compared to nz players it is relatively similar to when other players finished there test careers, Vettori (33), Crowe (33), Cairns (34), Parore (31), Astle (35), Fleming (34) Richardson (33). Really the only guys to play on the other side of 35 are Chris Martin and John Wright.
I don’t know the state of his back but he has been affecting his cricket since 2008, odds on it is pretty buggered by now. Presently he has a goal in mind being CWC which must be fuelling him. – but if he is not scoring the runs it will be hard for him to continue on
Right, but in those above situations
Vettori - not retired yet, simply can't bowl with his achilles injury.
Crowe - chronic knee problems and bad management forced him out early
Cairns - chronic knee problems, the ICL and County Cricket forced him out early
Parore - saw the writing on the wall with McCullum coming into the side, had business interests that were profitable
Astle - bad management and the ICL forced him out earlier than he'd have gone
McMillan - diabetes impairing his movement and vision forced him out earlier
Fleming - bad management post resigning captaincy and seeing a future in business/coaching got him out early
Richardson - couldn't maintain high standards due to not playing enough cricket and decided to give it away rather than damage what he'd managed to do.
Undoubtedly I think he's going to continue probably until he's 36 in tests. I think if anything he'd throw the ODI away after the world cup and focus on Tests and T20. That wouldn't surprise me in the least but ultimately, I think Baz sees an opportunity to do what Fleming did - take himself from a mid to high thirties to forties average, lead a team from the bottom of the table to a higher position and retire being seen as one of NZ's greats.
The other thing that will keep McCullum playing later than the others is the IPL, he knows a lot of his marketability comes from what he's done in International T20 cricket.