The way PayPal/X grew to millions of users is they gave $10 to each new user. This is effective marketing, especially if you make a fee on when they spend that $10. However, with exponential growth comes an exponential cost structure. And you cannot scale that which exponentially grows in costs.
So what is scalable? Exponential growth with a fixed cost structure. In software terms, it is exponential growth with O(1) constant costs, or at least near constant where the marginal cost of an additional customer is near-zero.
Amazon became scalable by turning variable costs into fixed infrastructure, then spreading that infrastructure over exponential demand.
In the beginning, their per-order costs scaled linearly, like warehousing, shipping, and labor. The switch happened decades down the road when they put massive fixed investments into fulfillment centers, logistics software (that improved at a rate where every additional customer made it more efficient), and data centers. They used a non-scalable business (retail) to finance a highly scalable one (AWS- fixed costs are data centers, spread across all users; Prime- fixed fulfillment center costs amortized over millions (billions) of customers).
So retail was a linear cost (not scalable, exponential costs as you get exponential growth, like PayPal- though they probably got some efficiencies right as they grew to hundreds of millions of customers), but it financed the creation of a scalable business model (AWS + Prime with fixed costs on data centers and fulfillment centers).