I don't think this is morally useful because it treats the issue of free speech as primarily legal in nature, rather than a principle by which to order society. But of course the only reason why it's a legal principle at all is because of its value to society.

As far as the logic goes, it is inadequately developed. For instance, you have as a premise (but only an implied one!) that there exists such a thing as "speech that destroys the Constitution" and another premise (implied!) that there is no practical defense save stopping the speech.

I explore the issue in more depth--as Popper's paradox of tolerance, but the logic for free speech is the same as it is one type of tolerance--in this article: https://medium.com/@ichoran/there-is-no-paradox-of-hatred-fbc3ddeb2500