There are already too many people who are being supported by fewer and fewer wage earners and taxpayers. The liberals would have the number of people being aided or even fully supported grow and grow. There are only a finite number of wage earners and taxpayers. Eventually, the number of people being supported will overwhelm the number of people doing the supporting, and then the entire economy is going to crash and burn.
The issue with extreme liberalism is the same as the issue with extreme conservatism in the United States. Both movements are so hung up on process and labeling that they're too busy fighting within their own little groups to be taken seriously. Neither will ever be the downfall of the United States.
America is a resilient country. When the pendulum swings to one extreme the country has a way of changing course and reversing direction. The political climate of liberalism will run it's course and America will find it's way back to an equilibrium of more conservative thought. Though liberalism is a very destructive ideology it has a way of waking the sleeping masses and setting the country back onto a footing of moderation.
The status quo clearly isn't working and something must be done. Conservatives are all about leaving things alone and letting things work out for themselves. Liberals want things done to improve the current situation. As such, extreme conservatives will be the downfall of America as opposed to extreme liberals. At least liberals want to try new things to try to better everyone's situation in America. Otherwise, extreme conservatives will simply widen the poverty gap even further with their policies.
I do not feel that extreme liberalism could be America's downfall, at least no more than following extreme conservatism. Liberal people propagate many useful things for society such as equality and equal rights. Without these tenets many of the forward thinking in American society would not exist. Liberals are no inherently bad, they are not trying to deconstruct the country.