I have to admit, occasionally I do dive solo. Most are just boat bottom cleaning, which are under water, but I don’t really consider "dives", as such. I’ve also done the occasional dive, for lack of a buddy. Seems to be a dearth of them in my area. Or perhaps it’s the fact that I’m 65 that scares off potential buddies. These dives are always what I consider minimal dives. Say less than 40’ depth. I know you can have strokes or other problems that could keep you from returning from one of these dives. However, I also drive solo, and ride a motorcycle solo, and sail offshore alone. The only difference I see is that the body is easier to recover except perhaps in the latter example. I’m not trying to be flippant here, just add my two cents. Also consider the skydiver, or solo aircraft pilot. Basically, we each determine our own risk factors and how far we will stretch them.
I also read the article mentioned above, and it is a good piece.