Become familiar with Pareto https://en.wikipedia.org/wiki/Pareto_principle
Some clients are more important than others, specifically by the income they contribute to you (now and/or in the future).
Excellent point. You always want to focus on the part of the job that has the potential to bring the most value with minimal work. If a single user wants a feature, it is better to be ignored in favor of more lucrative business opportunities within the business that could bring more cash and also value for customers. I will keep this in mind.