Third parties have an easier time fucking the Democrats when they're defending the White House because they have an incumbent president (Clinton, Obama) and their successor (Gore, Clinton) that they can attack as more of the same, continuation of the failed status quo, etc. Harder to pull off that trick when there isn't already a Dem in the White House.
If anything, I think a GOP primary challenger emerging against Trump is more likely than Jill Stein or Gary Johnson having their way with disaffected Millennials again.