Diving with a buddy (which you should absolutely always do) in good weather basically reduces risk to zero as long as you don't get too far away from each other. The chances of four regulators failing (two main, two octopuses) are close to nil.
…as long as you don’t penetrate a wreck, dive under ice, or dive in a cave. Since a roof severy decreases the chance to survive if something happens and you need to surface.
Also, of course, tech diving, typically below 40m, can also severely limit the ability to emergency ascend to the surface without injury.
Also as long as you keep a line straight to the surface. Cave diving and wreck diving are far, far more dangerous, and require special training. There are many ways to die if you can't just head straight up for oxygen in an emergency.
I strongly disagree that one should always dive with a buddy. But I’ll also say that one shouldn’t dive alone. Exit and entry are usually the most dangerous part of the dive.
Diving in low vis, spear fishing, or confined spaces don’t match with having a dive buddy. But they do match with redundant air supplies and other safety measures I mention in another comment.
Also unless your buddy is literally within arms reach at all times, they are not a redundant air source.
