According to IRS, the wealthy convert their economic resources into political resources deployed to protect their advantages. Past a certain threshold of inequality, the gap in political resources is so vast that the wealthy–by means of insidious think tanks and the Republican Party–basically capture the democratic process and effectively disenfranchise the rest of the population, leaving us with a de facto oligarchy hiding inside a meaningless husk of democratic forms. Serfdom!
Will pointed out that the empirical premise here seems questionable. Plenty of rich people vote Democrat, precisely because they are so materially secure that they prioritize their (typically socially liberal) values instead.
[This doesn't touch on the more serious problem of post-election corruption / regulatory capture. But there I guess the problem is not inequality per se, but the way that flows of money shape the legislative process. So the solution is to reform the political process, not limit wealth inequalities.]
We should of course provide a 'safety net' (e.g. a basic income guarantee). But I don't see such a need for a wealth ceiling.