I'm not a Pebble user, so I don't know how the app install process works, but can't Core just create their own store from scratch, not based on the existing app catalogue, and have that coexist as an alternative option to Rebble? Then users who want access to that extensive back catalogue can use Rebble's store. Let developers and users pick the stores they want to publish to and download from, respectively.
Given that Core is a commercial enterprise, it doesn't seem appropriate for them to rely on apps that were scraped from the original Pebble store. Core is a separate commercial entity from the original Pebble, and doesn't inherit the relationships between original Pebble and the developers which published to their store. By creating a store from scratch, Core can reestablish each of those relationships one by one. That would go a long way towards helping Core build back whatever trust they may have lost (it seems some users are still bitter about the original closure of Pebble, and I don't blame them). Otherwise, what you have is a commercial entity profiting off of a bunch of applications for which they don't own the right to distribute.
As a developer myself, I might be ok with my app being archived due to an emergency situation... but having that app be republished by a commercial entity is a red line.
I don't have an ethical problem with the back catalogue existing, but it should be hosted by a non-profit. Core can position it's store as an place for new, or updated apps that are being actively maintained by developers, which is definitely a selling point. Rebble can position it's store as a back catalogue of apps that existed on the original pebble, offered on an as-is basis. Which is also a selling point, because who knows what great gems you might find in there...