My husband did temporary construction work several times at Bethel, during the early 1990's. He said the elders he talked to, told him the GB seldom, if ever, go in service. Many of them are just too old and tired to do much of anything, and the others are terribly busy running that huge corporation. Others see themselves as too recognizable, like celebrities, so put in their time informally.
When I was there visiting, during one of hubby's work stays, I saw Karl Klein come out of his office. He looked to be about 175 (kidding) and very frail. I doubt he could walk very far. That was in 1992.