Answer:
In Economics, the phrase "All esle held constant" is also sometimes written in Latin "Ceteris Paribus". In Economics, this assumption is fundamental to the whole academic discipline since Economics is based on economic models that make a series of assumptions in order to reach partial conditions.
So in Economics, the reasoning is always in the manner of "all else held constant".
In Marketing, what is always done is to estimate demand for a product, and then, apply a marketing strategy in order to try to not only meet demand, but sell even more. This is because the main goal of Marketing is to satisfy customers beyond their expectations.