because there is a big difference between what is best for Abu Dhabi now, for what's best for Abu Dhabi when Sheikh Mohammed dies (saving illness, coup/assassination or accident, he will die in the late 2030s or early 2040s, when your brothers will also be old)
he earns nothing by appointing an heir now, he is waiting for his eldest son to gain experience and power
Candidates:
Hazza bin Zayed (only four years younger than MBZ), Deputy Chairman of the Abu Dhabi Executive Council (equivalent to Deputy Prime Minister of Abu Dhabi)
Tahnoun bin Zayed (seven years younger), National Security Advisor (head of UAE intelligence)
Mansour bin Zayed (nine years younger), Deputy Prime Minister of UAE
Khalid bin Mohamed (21 years younger), Chairman of Abu Dhabi Executive Office (executive branch of the Abu Dhabi Executive Council)
now, Tahnoun is the most powerful, Hazza is the most experienced and Mansour is the most popular, but opting for lateral succession can cause instability for the country, Khalid may be the best option
Yes, see the cycle. If Hazza, Tahnoun or Mansour become Crown Prince now, then later on if one of them becomes the President of U.A.E, they would want one of their son to be the Crown Prince of Abu Dhabi. So MBZ's son Khalid will never get the chance to become the Crown Prince or the President.