although I don't think it's for all divers. As someone said, DIVING isn't for everyone.
I made research dives in college that pretty much required solo work, but these were tended dives (but with only "line pulls" for surface communication).
These days, I'm into U/W photography. Even "with a buddy" usually another photographer, I am diving the SOB system (same ocean buddy). Watch any group of photographers on a boat and they are pretty much diving this way.
(If I am spearfishing, btw, I WILL NOT go solo, because of the entanglement hazards, among others....But, I usually free dive when I carry my gun, anyway)
I don't go usually go solo off charter boats, mainly because these are charters with RULES. But, I have had buddies on these charter trips that were probably more dangerous for me than diving alone.
Off private boats, I sometimes go solo, but I will not go without another diver on the boat. I think this is safer than having both of us go down and leave the boat untended. Boats go adrift or sink, and sometimes the current will change and force you to make an ascent away from the boat. It is more than just convenient to have someone on the boat who can come get you if things get to rough or if you are having a problem.
I've never thought about a depth limit, although I never dive deco or in "overhead" conditions. On my last solo dives, about a month ago, my max depth was 70', which is about the average max for me. I seriously doubt I would plan a solo dive much deeper than that....
Final thought: Solo diving is more dangerous than buddy diving. That's a given. But, I don't think it is more dangerous than many forms of deep, wreck and cavern diving, often practiced by those who think solo divers are taking unecessary risks and refuse to acknowledge it can be done with reasonable safety.
I understand the dangers and try to reduce the risks, but recognize the dangers remain. For some, solo divers are idiots (I used to think that way), but in the case of a properly trained and equipped diver, I believe solo diving is just another facet of our sport . (Improperly trained, equipped and experienced solo divers are still pretty dumb).