It is fine to initially build around a single source/sink, often you have to as you lack the resources to successfully be any more generalist, and often a new product/service evolves out of an interaction with that one entity (or pair of entities if the product acts as a bridge). The risk is staying in that state too long, and the trick is knowing how and when to start diversifying, or at least having a plan for diversification in case it is needed at short notice.