From the article: "On average, the marijuana users who participated in the study consumed the drug three times per day."
^ Of course there's a brain decline if you smoke it 3 times a day. That's being perpetually high. If you drink alcohol heavily 3 times a day you'll also decline pretty rapidly.
The UK does not tax worldwide income. In fact, if you hold your money in an offshore account, you can declare yourself a "non-domiciled" UK person. This special category exempts you from all tax on foreign income in exchange for a small fee.
It is not for no reason that every corrupt Russian oligarch and oil baron and African dictator flocks to London.
Your argument is predicated on the assumption that the rich use their wealth only for the "important forms" you mention. What you don't mention is how they are able to influence the political arena with their wealth. The influence of politics by a few is a very natural consequence of massive inequality, and is a zero-sum game. More political influence of the wealthy means less political influence of the poor.
^ Of course there's a brain decline if you smoke it 3 times a day. That's being perpetually high. If you drink alcohol heavily 3 times a day you'll also decline pretty rapidly.