Not just a monopoly, but also when power concentration becomes too much of a risk to the market.

If just one of the big banks were to fail it could take down the entire economy along with it. Even after all the stress tests... Because they don't count the interconnectedness of these backs.

For this reason even the Libertarian and staunch capitalists should be clamoring to break them up into smaller chunks. There's no reason why such an action should be bad for investors or the banks themselves. It could merely be a forced diversification.