Anyone who thinks the GOP will just die (both in Congress and in presidential races) because of how millennials vote has learned nothing from this election.
You're the only one who said that. What is actually being discussed here is a pretty constant theme for both parties throughout American history.
Even if there is a realignment at the presidential level that favors the Democrats for one or two decades (I doubt it, but okay), Republicans will still show strength in gubernatorial and Senate races. Even the House should remain competitive.
Thats because Democratic voters will be centered in large urban areas and states like CA, NY, NC, and GA. The Republican will also have an edge in senate and governor office because of this. The house will probably lean R unless they completely collapse in the suburbs. I thinks its to early to say that the next realignment will be the death of the Republican party but 2016 prove that the rust belt trend was faster than the sunbelt. But when it does we need to see the Republican strategy going forth before we talk about a permanent D majority.