Sean Trende's book "The Lost Majority: Why the Future of Government Is Up for Grabs and Who Will Take It" made the point that there are no permanent majorities in politics, because losing parties adapt.
The closest thing to "permanent" in American politics is the Supreme Court, and it belongs to one party for the rest of my lifetime. The Supreme Court's power is indirect, but it's absolute: the Constitution means what they say it means.
And that makes them the final say in who gets elected. They can nearly always find a reading of the Constitution as a fig leaf for the outcome they want.
That includes determining who will choose their successors. They don't usually apply that indiscriminately, but as of now there's no reason not to. There is no appeal.
Eventually the demographics may change who makes up the party. But as a matter of partisanship, the opposition is now irrelevant. Even if they have a short-term win next week, it won't suffice to change the permanent nature of the composition of the Supreme Court. It's different this time because there's no adaptation that can suffice to overcome that thumb on the scale of every future decision.
I agree parties have evolved greatly over time. The point I state in the article is that the Republican Party does not seem to be making any accommodation of younger and racially/ethnically diverse electorate that are now the majority of voting age adults for the time in a presidential election.
They don't need to adapt in policy. They can simply adapt in the mechanics. They've done their best to exclude those demographics from the ability to vote. Should any of those exclusions be questioned, they now have a large majority of justices to affirm that they are legal.
It doesn't matter that they're a majority. The circumstances have been rigged to allow a minority to rule, and having done so, to continue to set the circumstances to entrench that power. That move has finally become complete and it doesn't matter what millennials want.