Ubuntu seems to have made clear that the snap client will not be able to connect to alternative stores. What is the point of building an alternative store.
Further snap is controlled by one company. Canonical has a history of developing groundbreaking new things for the Linux community before unceremoniously killing them off.
* Ubuntu One * Unity Desktop * Ubuntu Touch * Upstart Init system
Do you see a trend here? There is a reason the community is skeptical of something completely controlled by Canonical.
Ubuntu One file syncing, also server part, has been Open Sourced with much effort from Canonical: https://ubuntu.com/blog/ubuntu-one-file-syncing-code-open-so...
Ubuntu Touch https://ubports.com is run by community and has rapid progress, it's running on PinePhone etc.
Canonical would have liked to continue Unity: https://www.omgubuntu.co.uk/2017/10/why-did-ubuntu-drop-unit...
I would presume Upstart Init system was changed to systemd because other distros started using systemd.
I don't see a trend here.