To put a fine point on it, what difference would it have made if only five witnesses handled and saw the plates rather than eight?
It is important to acknowledge how risky it was to enlist each witness if it was a fraud. Quite a number of people were witnesses to the Book of Mormon translation or the gold plates:
Who can you trust to never let slip your secret? It is difficult to imagine Joseph dedicating his whole life to the cause knowing that at any point one of his witnesses could have called him out as a fraud.
Why did Joseph choose to show the goldplates to eight people? Why not just two or three? Why did Joseph use multiple scribes rather than one the whole way through? When creating a fraud as big as Joseph would have needed to create, every witness is a liability. The greater the number of witnesses the greater the risk.