What does whither away are things like Private Property Rights and other elements by which Capitalist society maintains itself.
The “whithering away of the State” is one of the most commonly taken out of context aspects of Marxism, most people associate the State with all aspects of Government. Marx does not make that same association, and used the word State as shorthand for the aforementioned Capitalist elements of government.
This is why there’s a big difference between Anarchism and Marxism. Anarchists seek horizontal organization, and Marxists are fine with central planning and endorse it.