"when the Shopify platform makes the same set of selling advantages available to everyone?"

Shopify doesn't make advantages like branding, brand awareness, marketing, market research, etc available. It simply makes setting up an online store more convenient for people do. Convenience != competitive advantage. You can extend this argument to cloud platforms. AWS, and Azure make setting up and scaling a SaaS easier, but it doesn't give you any secret sauce that you need to succeed, like the ability to know what your customer want.