"It is not a slippery slope if you set clear boundaries." How could those boundaries ever be clear if they aren't near absolute? Definitions, interpretations,

"It is not a slippery slope if you set clear boundaries." How could those boundaries ever be clear if they aren't near absolute? Definitions, interpretations, and the cultural mindset change constantly. Prejudice today is different than yesterday, and will change again tomorrow. I see a remarkable tendency of people to assume that liberal / progressive ideas will always remain free to express, so there's no need to protect all speech. I don't see how this is true.