The advantage of a 401K over the Roth is that since both can be raided, at least with the 401K your money is being taxed only on the way out. With the Roth you are being taxed on the way in for sure, and you run the risk of it being taxed on the way out as well.
Judging from the past my guess would be that we wont see something as obvious as a penalty or a fee. Instead what you may see is a more aggressive required minimum distribution schedule. Perhaps you could see the date of penalty free withdrawal pushed back from 59.5 to 65 or 67 years of age. Means testing of social security benefits has been mentioned before. If that gets put in place, and you fail the means test for social security, they will have effectively taxed you more your entire working life.
Or we could just continue doing what we are currently, and just keep piling up the debt :) No tax increase or Roth/401k raids needed.
Don't forget the more politically expedient (and in my view most likely) choice: Print money to pay for it, causing inflation, which is essentially a silent tax on everyone with money.
Take a look at any of the Millenials working in any large-city downtown: they spend every single penny they earn, they have no savings; when they get older, either the State will let them starve to death, or taxes will be raised in order to take care of them.
I don't think the former is politically possible, so taxes will have to go up.
Meanwhile, the same spendthrifts are by-and-large refusing to have children early, which means that there will be fewer workers to pay those taxes.
That means that there will be tremendous political pressure to raid 401(k) plans and IRAs. I actually believe it's even odds whether Roths will end up taxed one way or another (perhaps with 'withdrawal fees' or some form of Social Security clawback or something): it'll be too much money for the State to leave alone.