Since I'm approaching a decade of free speech being at the forefront of my priorities, I think I've made and heard just about every argument for the absolutist view, and heard every argument in opposition.
There really aren't any good arguments in opposition. They all fall like a house of cards the moment that anyone examines the logic or the history.
The argument in favor of free speech absolutism, that I think is ultimately impenetrable, is as follows...
Imagine there is a beautiful truth that every good person objectively agrees upon. Every person who means well will speak this truth from the hilltop through a bullhorn with no fear of a valid, dissenting argument from another well-meaning person.
In fact, it's demonstrably true that any dissenters to this beautiful truth are bad-faith actors. They're all liars and manipulators. Also, imagine that we can actually silence them. Imagine that any dissent to this beautiful truth could be stamped out.
We still lose.
We lose because we'll forget the bedrock of the beautiful truth if we don't need to rigorously defend it. If we lose the foundations of our beautiful truths, they're bound to crumble.
I haven't met many Holocaust deniers in my life. I've met dozens of people who accept that the Holocaust happened, but couldn't tell you how they know it happened to save their lives. I've only met a couple of Moon landing deniers. I've met dozens of people who were perfectly willing to chastise Moon landing deniers while not being able to explain how they know that we did land on the Moon.
Knowledge is similar to a physical structure. If it's neglected for too long, it decays. It must be reinforced to maintain its beauty over time. We must be reminded to reinforce our truths from time to time.
We need the fat white guys in pee-stained, white bed sheets, spouting a bunch of monosyllabic, eugenic clap-trap, while claiming to be the master race to remind us not that racism is stupid -- but why racism is stupid.