I don’t know that any group (including blacks) will “always be Democrats” because even as white Americans dwindle in numbers the lives of the Dem core (blacks, Hispanics, women, sexual deviants) are hardly improving. In fact, those groups are realizing they have to grow up and pull their weight as white America abandons whole swaths of this country to them. Like Tolkien’s elves, they are leaving...and there is nothing those who relied on them for food, shelter, public education, road maintenance, etc. can do about it. They either grow up, or their neighborhoods devolve into those from which their ancestors came (Latin America, Africa, etc.).
I suspect the push to integrate women and sexual deviants into military units is due to something along these lines; as whites have fewer children, I doubt they will risk them in the bizarre half-hearted adventures around the globe in which our governments involve us. I personally couldn’t imagine my children wearing a uniform for any other purpose than defending the shores of the United States itself, and I pity those parents who’ve lost their children in wars our present government won’t prosecute seriously.
So Trump will need to get an ever larger share of the white vote to win. He can do that if the democrat turnout is depressed, so while the raw vote number goes down, he will get a bigger percent.