> It would also be entirely free
This can’t be true, since no matter how you create the central sales channel, some sellers will get preferential treatment in terms of screen placement, appearance in search results, etc. Shopify could not pay the costs of operating and advertising that central channel unless the most successful businesses are placed more prominently and lead to higher conversions of some kind (sellers converting, leading to less churn or more subscribers, etc.). But every Shopify customer will want that - so who gets it? If you do nothing and “the rich get richer” and there’s no way for an outsider to break into the better display rankings of the sales channel, you’ll just alienate customers and see a huge drop in new subscribers. This is often a big problem for ecommerce companies that facilitate online sales portals like Amazon, eBay, Etsy, and even more niche things like Shutterstock.
You either “democratically” allow customers to pay for placement in your central product index, or you force customers to pay via lost business and lost opportunity, which they’ll be embittered by. But there’s no such thing as a “free” way to centrally index across all hosted subscribers.
> “GoDaddy, Wix and Squarespace couldn't really afford to do it because they aren't e-commerce focused.”
I think you are very unfamiliar with the hosting industry. All three of those businesses make the strong majority of revenue from subscribers of ecommerce plans and all three have huge platform offerings and nationwide advertising campaigns targeted specifically at ecommerce customers.
All three of them practically only exist (in terms of revenue) because they are a good hosting option for small businesses that sell online.