It should not always be the number one option for millenials to put all their money to bitcoin. I could agree in half funds, but there still a risk of losing an amount especially if these millenials have a very little control over their emotions. They could be affected by FOMO and FUD. Meaning, even if we know that Bitcoin is good for long term, its volatility vould induce losses if they execute buy and sell of bitcoin prematurely.
Millennials don't really think about security just for the moment, they don't also have much savings and savings isn't even a culture yet from what I know from my own circles of friends. I'd never tell them to put it all in bitcoin though, gold and real estate are still the things that for sure stick around so yeah, diversify your risk, people.