> What says we will not continue to accept more and more, as long as it is only a small step from status quo?

The Overton Window [1] says this is exactly what will happen.

There's a great video [2] about how this applies to Trump (which is where I first heard about it) and gives a good example of what "shifting the Overton Window" looks like.

[1] https://en.wikipedia.org/wiki/Overton_window

[2] https://www.youtube.com/watch?v=_v-hzc6blGI