Millenials enter adulthood at a time where the world seems upside down in a variety of ways. Millenials witness war, massive country debt, declining jobs, rising immigration, and a whole host of other issues that causes uncertainty in their minds. Millenials seem to trust the money that they can quickly access and seem as afraid to not access the money on a regular basis.
I think that the millenials love cash because they feel insecure about their job prospects. I think cash is smart when you're underemployed or unemployed. I think when the economy gets better and they start worrying about their retirement then they will appreciate the gains that the stock market offers.
I think even though Millenials love cash, they still will invest and try to make money off of the stock market. I think they will see how it can be profitable and try to make the most off of it. Even though someone likes cash doesn't mean they would pass up on a good opportunity.
Millennials love cash, but they also seem to be willing to work as much as their predecessors, despite the belief that they are coddled. They are masters of working efficiently, rather than traditionally, and they will excell at playing the market. They will be more innovative, and focus on how to make the stock market work for them.