When a product is stable enough to expand
Expansion is the last stage of the journey, and it is a decision, not a reflex. The decision is which axis to grow along, but only after one product runs without constant intervention. Stability has three tests. Rank holds through normal demand swings, rather than sliding the moment advertising eases. Margin survives a full review cycle, including returns, storage, and the fees that only show up over a defined window. And inventory no longer surprises you, because reorder timing and demand have become predictable.
A product that still fails any of these is not a base to build on. It is a problem you have not finished solving. Expansion will not solve it. It will duplicate it.
The three axes of expansion
Growth from a stable base runs along one of three axes. Each adds a different kind of exposure, and each spends a different resource. So the choice is really a question of which resource you can spare.