In the USA, white Conservatives consistently have the highest voter turn out.
But our advantage shrinks every year.
A majority of the people who die each year are white Republicans.
A majority of the new 18 year old voters are non-white, and they vote 80% Democrat.
Young white voters vote like their parents - 50% to 55% Republican - but a large share goes Third Party.
Since Trump got elected, we have 1.5 million new Naturalized citizens - they vote 80% Democrat.
Adult Hispanics and Asians vote about 65% Democrat. Blacks vote about 90% Democrat.
Bottom Line....
If the Democrats get their voters to polls, we will lose the House and the Senate.
If Republicans turn out in exceptionally high numbers, we can hold Congress, but just barely.
2020 will be the turning point. Win or lose, Trump will be the last Republican president, and the Senate majority will turn Democrat.
After the 2022 Congressional Redistricting, the House will be gone, too.
Political Epitaph for the Republican Party....
“30 Years of Massive Legal Immigration”
Your scenario is a possibility. Demographics is destiny as Mark Steyn says.
But you left out the shooting parts.
Though I share your pessimistic outlook, one factor that could delay permanent Democrat rule is that as the Democrat party becomes ever more explicitly anti-white, the percentage of white voters going Republican ought to increase.