I'd say it wouldn't make sense to do this. Shopify has no relationship with the retail customer. To build it, they'd have to build traffic from scratch. They'd have to not only build a unique customer brand, but also have to figure out a way to drive traffic, which would likely be through aggressive marketing (Facebook / Google ads), which wouldn't be economical. Secondly, what would Shopify build a brand around for customers?
Amazon's brand is built on the promise of not only providing the cheapest prices, but also the convenience of 2 day shipping (achievable only through scale), amazing customer service (and lenient refunds, again achievable only through scale). This being said, what would Shopify's "brand" promise be?
Curation of their favorite products? Who says that Shopify is qualified to curate?
All of these factors make it hard for me to believe that they'd enter and create a serious business from a unified e-commerce site that'd compete with Amazon.