Don't be daft. There are too many contingencies to make that kind of an assertion. You are mashing together today's politics with the events of 150 years ago with little regard for all the little turning points that came in between.
You also ignore that it's in large part precisely the Southern influence that drove Northern states away from the Republicans in recent years. When Republicans became the Southern party to a lot of Americans they turned away from the party.
It's also jackassery to assume that other Western countries that an independent North might possibly resemble are "full fledged Marxist." Whatever you think of Britain or Canada or Australia, they are far from Marxist.
One might as well say that without the North, the South would be a fully-fledged socialist country by now. It might be a different kind of socialism, but socialism none the less.
Look back on the New Deal South -- Huey Long's South, George Wallace's or Lyndon Johnson's South -- plenty of opportunity for socialists of a populist sort there.