Today, the Governing Body can make any arbitrary decision it wants. It doesn't need the approval of Scriptures to change policy or doctrine. The advantage of claiming "new light" allows it to make a 180 degree turn without being answerable to anyone.
But for the sake of unity, it probably came up with this new ruling of 2/3rds approval so that issues would not come to a standstill. Undoubtedly, it is based on practical considerations rather than Scriptrual ones. Having experienced a GB member who, at times, opposed the majority opinion (Ray Franz) it may have considered the necessity of this arrangement.
This 2/3rds policy reflects more upon the reality of the organization than one realizes. It is a corporation to a world wide publishing empire...not a scriptural Christian organization of believers. While the GB has turned over the running of the day to day affairs to committees, the GB remains the head and heart of this large business corporation.