No, actually. There is no slippery slope that inevitably leads to all speech being illegal.
Instead, the court system in the USA mostly does a pretty good job determining what is or is not speech, and ways of having very limited restricts on speech that only prevent the extremely harmful stuff, while maintaining people's rights.
Addressing these "corner cases" is exactly what the court system does, and it has resulted in a society where the vast vast majority of speech is protected, and only very extreme and damaging edge cases are banned.
Instead, the court system in the USA mostly does a pretty good job determining what is or is not speech, and ways of having very limited restricts on speech that only prevent the extremely harmful stuff, while maintaining people's rights.
Addressing these "corner cases" is exactly what the court system does, and it has resulted in a society where the vast vast majority of speech is protected, and only very extreme and damaging edge cases are banned.