He's comfortable that we could, so far as the evasions of the rich are concerned -- we've got them, or can get them, trapped. And, while it's curious that he seems to understand that taxing the rich can cause "great economic damage", he thinks that as long as the marginal rates are below historic highs (or, as he says, below what they were "at the beginning of the neoliberal era"), such damage won't at least be "great". Why he should assume that economic history can be simply run backward in this way without great, or at least unforseeable, damage he doesn't say; why he should assume that economic damage great or small is a worthwhile policy objective, again he doesn't say -- it's almost as though seizing money from the "well-off" is a good thing regardless of a little damage.
In any case, the real risk, apparently, isn't evasion or damage, but politics: "The big problem then, is the politics." Why, one wonders? It obviously isn't because the well-off voters outnumber the non-well-off -- Quiggin is so far only proposing to loot from the top quintile, leaving 80% to split the proceeds. Could it be because he worries that the 80% might actually have some moral qualms about this, might in fact think that simply taking other people's money isn't far enough removed from the ethical level of a common thief to suit them?
Nah -- silly me. It's all just because "The political power of the well-off and rich has increased massively over the past three decades, and (with the arguable exception of the finance sector) has not really been diminished by the crisis." So much for democracy, but then democracy was never much more than a farce in the eyes of the "anti-capitalists" anyway, was it?