Buffett notes in various shareholder letters that the managers work because they love the work, not necessarily for the money. The subsidiaries of the company also operate with significant autonomy. These managers are not subject to the quarter by quarter scrutiny of Wall Street who may or may not have been their former masters. Instead they can concern themselves with the long-run viability of their respective business.
Should Buffett's successor take a more hands on approach, it would be possible some might rather walk. I don’t see this happening though. The board of directors which oversees Berkshire is unlike most corporate boards. As Buffett noted in this years shareholder letter, the board does not get to expense D&O insurance on the company dime. This type of insurance pays directors and officers for legal expenses related to wrongful acts the firm had taken under their watch. It is considered one of the more expansive forms of insurance in terms of coverage as it is generally paid not by the directors and officers, but by the shareholders.
Berkshire does not permit this hazardous activity. The directors will have their feet firmly to the fire if Buffett’s successor runs the company into the ground (Bill Gates will doubly be so as his charity becomes
The other concern Stifel Nicolaus mentioned was the successor may be less nimble at deploying capital than Buffett would be. This is harder to dispute and much more obvious. Buffett’s knowledge of industries and businesses is encyclopedic. Being able to mentally retrieve the information and numbers like he does offers a significant edge when markets and prices move quickly. It would much more plainly be said that it will be hard to replace one of the greatest investors of all time.
Disclosure: Long Brk-B