There are principles and once those get principles get trampled in the name of expediency, it becomes easier for the next person the trample them further.
And then the US lost the war, went home in disgrace, and the promised cascade of falling dominoes never happened.
Sometimes resistance can make a slope less slippery. With Donald Trump’s election in 2016, I believe the US was on a slippery slope to a very bad place. A lot of stuff were being normalized that should not be (overt racism). However, there was a lot of opposition and resistance which has probably helped stabilize the situation.
So no, just because a lot of resistance keeps you from sliding down the slippery slope, it does not mean the slippery slope is not real.