Modern libertarianism has multiple roots. Libertarianism is alone among 20th-century secular radicalisms in owing virtually nothing to Marxism. They largely forgot about individual freedom, especially economic freedom, and nowadays spend most of their time justifying higher taxes, bigger government, and more regulation. Libertarians call this socialism without the brand label and want no part of it. For starters, by not being conservative.
Most libertarians have no interest in returning to an idealized past.
Libertarians want to abolish as much government as they practically can. Also, current libertarian political candidates recognize that you can't demolish a government as large as ours overnight, and that great care must be taken in dismantling it carefully. For example, libertarians believe in open borders, but unrestricted immigration now would attract in a huge mass of welfare clients, so most libertarians would start by abolishing welfare programs before opening the borders. Progress in freedom and prosperity is made in steps. The Magna Carta, which for the first time put limits on a monarchy, was a great step forward in human rights.