What motivates the GB? My theory is this:
The modern-day GB arrangement established in the mid-1970s was in order to prevent a loon like Fred Franz from becoming a one man show like Russell and Rutherford were. They wanted to both share in that power and attenuate the authority of the Society president. Then, in 1980, a faction of the body decided to remove any members who might take up the mantle of Sola Scriptura and place the Bible above the Watchtower publications and those who run the organization. At the time, it was Ray Franz, and any who may have sympathized with his view were frightened into silence by the ease at which any one of them could be cast out.
That generation only has one (or two?) members remaining on the GB. The rest, I believe, are likely a strange mix of true believers and guys who get off on the lifestyle and power. Once the old guard finishes dying off, who knows what will motivate the GB?