Wait another 10-20 years. Eventually the Democrats will wear out their welcome. There's also data suggesting that the post-Millennial generation (born in 1996 or afterward) is already trending more conservative than the Millennials, and it's likely that Millennials themselves will become more conservative after a while, considering it's very difficult to be more liberal than they already are. It could happen in 10 years, or it could happen in 20 years, but it will happen.
Well it's certainly a possibility that they could become a little more conservative as they age, that doesn't necessarily mean they will begin voting Republican, either. Remember, older Republicans who went through over a decade of failed Bush wars and a collapsed economy under his watch didn't really turn against him for longer than 4 years. You'd be surprised how deep party loyalties can run. Especially when the other party is so ideologically different and even counter to their beliefs.
Regarding how liberal Millennials are - Yes, and it's because of that that Democrats could actually lose some of them and still come out on top because of just how Democratic-leaning they are. If their party loyalties stick, which it looks like they are so far, then they can lose some and still maintain strong majorities throughout the country in many places.
