I'd disagree with that premise as I don't think the real reason for the trend left is that we have more young millenials, that millenials are more motivated or influential, or even that millenials are the squeekier wheel.
Rather I believe the main factor in the shift is the advent of the internet where our source of information is suddenly much more global, or at the very least you are much more likely to be exposed to fully informed viewpoints from every side of a given issue. This really is a big difference from when you mostly relied on your parents, teachers/coach/church leaders, the 2-3 major media news sources, or pop culture icons.
I agree that the "trigger warning" and "politically correct" (and backlash) phenomena will get old. As well as public temper tantrum "movements" like Occupy or Tea Party will get old but there have always been those on each side who think that if they shout the loudest they get to control public discourse. Right now it just happens to be the smaller subgroups that are getting their 15 minutes of fame.
I tend to think that underlying core of hippies was hedonism and not liberalism. Under that light it isn't at all surprising that they've grabbed as much for themselves as they can, rest of the world be damned.