I don't really have an issue with solo diving.
I have dived solo, and will again I'm sure.
There are however two observations.
1. I have always been a member of a club, which is a norm' in the UK, but appears unusual in the rest of the world.
As such, we teach our own divers, buddy them, take them diving, build their experience.
Diving on 'club' trips, we follow the rules of our agency; buddy diving, occasionally diving in groups.
2. There is a miss conception that PADI OW, or AOW means an experienced diver. Unfortunately, in most cases this is far from the truth.
Solo diving requires a certain mindset, and I would suggest, a minimum equipment configuration. A lot of those who ask about it, don't take account of those two factors.
There is another issue. The risk of legal liability if you are in a group (even if you are not part of it), and someone dies. If they are diving outside of the 'agency' recommendations, and you didn't interject, you can become embroiled in legal issues. This happened in Malta not so long ago. You don't have to eventually be found guilty of any wrong doing to have it really **** life for a while with legal costs etc.
I have had the unfortunate experience of dealing with the authorities in three fatality cases.
Once in France, once in Malta, and once in Scotland. Very quickly, one of the questions to answer was were they diving within recommended practices. Did they break procedure. What if any 'rules' where broken. Who was the most experienced person and what did they do ... or not do.
In some jurisdictions, you have to prove innocence, rather than they prove guilt! (Napoleonic legal frameworks).
One country I would hate to get embroiled in a legal case would be the USA. Where law suites seem to be thrown around with impunity. Insurers in the UK require you to ask for specific cover for the USA, because litigation is so expensive in the USA. (Similarly for medical cover).