One question that tells you if a growth move is defensible
Before you call an advantage durable, name what a bigger rival would have to give up to copy it. If the answer is nothing, it is not an advantage.
The test takes about a minute. Describe your growth move, then ask what an incumbent would have to sacrifice to match it.
That framing comes from a piece by Bruce Eckfeldt for Inc., which argues that most ideas labelled disruptive are nothing of the kind.
The logic is simple. An advantage only holds if copying it costs a larger competitor something they are unwilling to lose.
Work through the answer honestly. A rival might have to cut prices on a product that funds the rest of the business, annoy a distribution partner, rebuild a process their staff know well, or admit that an existing line was overpriced.
Those are real sacrifices. They explain why a bigger company can see exactly what you are doing and still decline to follow.
Now consider the other answer. If the incumbent would have to give up nothing at all, then your move is a feature they can add whenever it starts to matter to them.
That does not make it worthless. It makes it temporary, which is a different thing to plan around.
The distinction matters most when you are deciding where to put money. Spending hard on a position a competitor can copy in a quarter is a marketing cost, not a moat, and it should be budgeted like one.
It also matters when you are telling the story to investors or a board. Naming the specific sacrifice is more convincing than the word defensible, because it can be argued with.
The uncomfortable version of the test is the one worth running. Take your best current advantage, write down what the largest firm in your market would lose by matching it, and see whether the sentence survives being read aloud.
If it does not, you have learned something cheaply. If it does, you know which part of the business to protect first.