In business, even the idea that things will stay the same is just an assumption: present trends will continue only until they do not. Challenging your assumptions is vital, but there is not enough time in the world to challenge EVERY assumption. So when (and how) do you challenge assumptions?
Ask why, and then ask the why behind the why.
I have done a video about this, with insights from the theory of constraints and the 80/20 (Pareto) Principle: