You don't back down from a good thing just because it might be hard to implement. So then it just comes down to whether you think removing speech that incites violence is a good think or not. If you think it's good in itself but could be abused so maybe not do it? No, you do it and deal with the hard questions that come along every time you make those decisions.
Well, what if the definition of "we" changes? What if you become the minority and the majority wants your speech limited?
The only reason why we have things like gay rights, women's rights, civil rights, etc, is exactly because of free speech. The speech at that time were the minority but were protected by the Constitution.
If we don't respect the rights of those that we disagree with, when (not if) they come into power, they will use those same rights to quash our free speech. And remember that almost half of the US voted for Trump who is demonstrably the worst president in US history. If there is a smarter, more palatable right-wing candidate (ex. Tom Cotton, etc), we really could see a Republican president in office by 2024. And they would use that hammer that the left have been trying to use to quash that free speech.
We have the Constitution and Free Speech for a reason. And it has worked for centuries now.
Even then, for something like resisting dangerous speech, what does the slope matter? If the original thing is good to do, don't worry about the slippery slope, just make the right decision. Then of it goes to far and you're on the edge of a slope, that's where you fight.
If guidelines about violent speech are abused, then fight to fix it. Don't avoid the correct decision just because doing it right will be hard work.