From your link, I think the author nailed it. The fear is that the current leftist ideas are tomorrow's mainstream ideas and tomorrow's left will be more extreme. The reality is that tomorrow's left will have opinions like the NYT writer. You can see the inverse of this on the right, too: Rand Paul has surpassed Sarah Palin; Alex Jones surpassed Glenn Beck.
Edit to add: So the progression of political trends isn't linear. Liberals push for the next new idea and conservatives want to hang on to (or revive) some other value, but (hopefully) neither is going to endlessly push their agenda today until it's some authoritarian mockery of itself.
Edit to add: So the progression of political trends isn't linear. Liberals push for the next new idea and conservatives want to hang on to (or revive) some other value, but (hopefully) neither is going to endlessly push their agenda today until it's some authoritarian mockery of itself.