@older @ilumium
The Guardian picked up that report from Dagens Naeringsliv (Business Today), which has a well-recognised neoliberal (small state, low tax) bias, so I'm not sure how reliable it is. In France, the general experience of wealth and other taxes on the super-rich is that they and their media friends make a lot of fuss, threaten to leave, a few leave, most of those fairly quickly return. A Europe-wide agreement would probably further disincentivise this reaction, as people would have to go a long way to escape it.
But the arguments in general for taxing very high income and wealth levels heavily are pretty comprehensive in my view.
For example:
1. The only period capitalism has worked reasonably well in Western Europe was in the 30 years c.1945-75, when in the aftershock of fascism, war and holocaust it was recognised that rampant inequality had led to social division and ultimately disaster, and countries built welfare states with very high redistributive marginal tax rates - which far from curtailing investment and growth seemed in fact to do precisely the opposite.
2. It's better for everybody - not only in the 'Spirit Level' sense that everybody, even the rich, are happier in more equal societies, but because it is the most just and effective way of controlling inflation.
3. And then of course it's better for the environment, because excessive income and wealth in private hands is a main driver of over-consumption.
Note these are all pretty factual points - there is also the 'natural justice' argument !