Manofman, the reason your point is off kilter is that if an aircraft has a decompression or loses an engine, it will be making a landing at the nearest suitable airport anyway. The rate of accumulation of engine deposits is unlikely to further affect that flight, and the aircraft will be undergoing repair for the failed system so boroscoping the engines won't be much of a hardship. Normal ops planned in a significantly contaminated environment are different because they will have vastly greater exposure and the aircraft will be expected to continue in service.