Walter once bubbled...
I never encourage solo diving.
I recognize the rights of adults to make choices for themselves, those choices include the decision to dive solo.
A small percentage of divers can dive solo with minimal risk.
The argument that you can learn solo diving in 2 days if you can learn to dive in 3 does not hold up under examination. I've never seen or heard of a 3 day OW course that I believe adequately taught anyone to dive.
I live in an area where solo diving is very common. Conditions are frequently poor - high seas, low viz, etc. Few of these solo divers have training beyond OW, few have equipment any different than what you'd see in a typical OW class (not counting the speargun and stringer). These folks make hundreds (thousands?) of solo dives to depths exceeding 130 ft every year. Many of them are divers who really aren't ready to be diving solo. OTOH, problems are extremely rare.
I'll dive with practically anyone, but I do so knowing I'm increasing my risk. I figure my risk is lowest when I dive with some specific buddies I know very well, next lowest when I'm alone and highest when I dive with most of the divers I run into on boats around the world.