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.