One of the downsides to hardcoding snap to only be able to use a single repo/store is probably added difficulty in creating testing infra for testing if uploads/CI/CD work.

lol, one of the first one’s I click on: snapcraft.io/test-snapd-public (by Canonical)

A basic buildable snap that is expected to be published in public mode

Maybe if they didn’t insist on holding a monopoly over the store, they would be able to have an internal version of the store for testing, rather than cluttering the public one.