See I agree with you, but can understand the opposite view. In MTB and CX, apart from the start, you never have 200 guys riding within 2 inches of each other and never at 30-40mph. So discs will only help on long wet descents and not when the bunch are rolling along the flat.
As for safety, they're about as dangerous as spokes and nowhere near as dangerous as an exposed chain ring, but apart from Spinergy wheels in the 90s, spokes aren't banned, even today's bladed aero spokes are fine. But somehow discs are an issue