I quite agree, personal use is not possession with intent to supply, which attracts a higher punishment tariff. However the bar is set pretty low for possession with intent to supply vs personal use quantities. Now that UK police have higher priority items to deal with I agree that the number will be low and will continue to fall.
I think it would be approved. There is no known link to lung cancer by smoking cannabis as long as it is not mixed with tobacco. Cannabis can be ingested removing smoking altogether and is the primary method used for medical reasons in the UK. Cannabis doesn't cause people to commit crime either, probably has the opposite effect, unless driving is involved, cannabis doesn't kill, there are downsides for people who are susceptible to certain types of mental illness, but equally it appears to help people with certain types of mental illness, there are downsides for heavy users, the drug itself is not physically addictive although some people may have a psychological addiction to its effects. On balance I can't see a reason why it would be prohibited if discovered today.
Actually, the usual responsible medical approach to a new substance such as cannabis would be a precautionary ban on sales, because we know there are serious negative side-effects but we don't know that we have all of them identified and quantified. Research has been limited and politicians have to take some blame for this. Nor do we know that the medical benefits could not be obtained by other means (including use of refined cannabis derivatives) without harmful side-effects.
However, when new substances of any sort are submitted for approval and rejected, its rarely a police matter, especially not possession. So while I think we need to avoid legalisation as a precautionary measure and in order to avoid implying approval or absence of risk, I wish the damn criminal justice system wasn't involved at all. I'd rather this was dealt with by civil regulatory authorities than cops. I bet the cops wish the same thing......