I agree, I don't know anyone offhand who uses two computers for recreational diving, and I would absolutely use an Apple watch as my sole dive computer. If you have a computer malfunction you can still safely return to the surface. The only reason to perhaps carry a backup is if you've traveled a long way or otherwise invested a lot of money in the trip.
Every dive I've done has been on a boat with quite a few people all doing the same sites during the day. If your computer failed you can still estimate NDL from others on the boat and just err on the side of caution. Or even just rely on paper charts like people did for decades.
If you're doing cave diving then obviously a whole different story.