I wanted to comment on this because it makes an assumption I see many people make, and I don't think it's sound. You say that the number of bigots are constantly dwindling as society moves past it and, as a result, they're basically harmless because nobody will buy into it. I see no reason to assume that society is on some inexorable march towards ever lower levels of bigotry.

Looking back on history we see countless examples of cultures becoming increasingly tolerant and then sliding back into bigotry. Take the KKK for example. The popularity of that group and the ideology that gave rise to it have ebbed and flowed throughout American history. Bigoted attitudes towards immigration is another great example. Many of the racist fears of South and Central American immigrants you hear these days echo the same kinds of things that were once said about Germans, Scots, Irish, and Italian immigrants at different points in history. We might look back on the treatment of the Irish and Italians and marvel at how intolerant Americans were, but as soon as you have a new massive wave of immigration- we're right back to those attitudes. For the sake of brevity I'll end the examples here, but in general, I'd say there really isn't an overall arc of history driving towards diminishing bigotry, I think it's more of an ebb and flow.

And this is why I don't think we can simply dismiss the potential harm of hate speech. Most people aren't born as bigots. Like any other belief, that bigotry is learned. Allowing hate speech in the public square can allow that bigotry to spread. Now, this does not necessarily mean that hate speech needs to be criminalized and excluded from the public square. As an American, I tend to be pretty extremist and object to laws that would forbid "offensive" groups from speaking. However I do find merit in the arguments of those who favor criminalization of bigoted hate speech. I disagree with those arguments- but mainly because I see the potential harm from criminalization to be greater than the potential harm of non criminalization.

In short, I just don't see a compelling reason to assume that bigotry will inevitably die out on its own.