>The reason for why the backend for the snap store hasn't been opensourced has been explained multiple times. Namely that it would be expensive to open source it with little benefit in return.

Perhaps a business reason left unsaid is that an open source snap store would undermine Canonical's paid "enterprise edition" snap store. Why would an organization pay $30000 (https://ubuntu.com/internet-of-things) for a private app store if they could easily set up their own snap store?