The Federal Government only has authority to tax income. There is no such thing as an ad valorem tax on investments. That is what the OP is pretty much proposing.
However, the argument has been made that a budget crisis would cause the Feds to raise the tax on distributions from 401k plans to something astronomical. I know a guy who does not contribute to a 401k or any other tax deferred retirement plan because he believes that this is going to happen. But he is an idiot because he is getting taxed twice and the hike on taxes for 401k plans would have to be off the charts to offset the double taxation incurred by going outside a 401k plan.