Let me offer an alternative explanation that I think much more likely than this deeply politically driven hype from the firms servicing this declining market.
It costs to be a non-dom. The charge can be as much as £60,000 a year. It increases the longer a person claims to be non-domiciled.
My suggestion is a simple one. The non-doms might not be leaving. Instead I suspect they may just be choosing to pay their taxes in the UK.
Because £60,000 a year is a lot of money people will volunteer to pay more?