My guess is that they will try to integrate Shopify sites into the platform, to give the FB Shops platform some momentum, then gradually push the non FBS stores down the page. My reasoning is that they are emulating Google.
Alternatively, they could be looking to make FB Shops an advertising platform, similar to Amazon Seller Central, where they don't care who you buy from, but they want each seller to pay for ads and/or payment processing.
Although being a simple shopify drop in replacement would have its advantages in simplicity, there is probably more long term potential if they embrace multi-channel sales, and become a storefront management tool ala https://www.ecomdash.com/ and https://www.shipstation.com/ Some people are going to keep shopping on amazon, some on ebay, some on walmart, some on etsy, some directly on websites.
Im not saying that's what they are doing, but its a way to skim off all transactions instead of trying to force business into facebook.
Probably the right answer. If you have a shop that's getting some sales on etsy, amazon or ebay or wherever else you're not going to want to close it down in favor of facebook – you'll just open a second storefront there.
Your guess appears to be wrong. I couldn't find anything about FB Shops using Shopify platform. Only that you can stock your store from the Shopify store.. which is a tool that Shopify provides. In other words, Shopify is using the FB Merchant and Store APIs. Like anyone can.
It seems like a bad deal for Shopify sellers. They must pay FB 5% and Shopify 2% of each transaction. If they jettisoned Shopify, they'd just pay FB 5%.
Alternatively, they could be looking to make FB Shops an advertising platform, similar to Amazon Seller Central, where they don't care who you buy from, but they want each seller to pay for ads and/or payment processing.