A 401k is just a legal entity. It's like forming a corporation or a trust. In fact, a 401k is just a trust with special rules -- rules determined by the government, and yes, trial balloons have already been floated to tax these supposedly "untaxable" trust account.
Argentina has already done it. No reason to think the current lawless US government won't, too.
The assets you put into the 401k trust depends on what you think about the assets -- stocks, bonds, precious metals, real estate, oil, currencies, derivatives, etc.
It is probably best to at least have some assets outside of government control if at all possible. Anyone with assets owned within a government created entity is going to be subject to the whims of that government. And yes, IRA's and 401k's are likely to be robbed by the government at someone point if things continue going the way they are now.
As far as the stock market goes, it's all about the FED printing money. When that stops, look out below. Until then, "irrational exuberance" can get even more irrational.
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