Having separate (read-only) repositories would help bundles developers a lot by making them require only our API/SPI interfaces when needed.
Typical use case is the REST client in PHP. It's completely pointless to make it require the whole ezpublish-kernel repository.
We can get inspired by what Symfony does with their components. Everything is committed in the main repository, but components are accessible separately thanks to synchronized repositories.
Example is HttpFoundation:
- Where it is in the main repository: https://github.com/symfony/symfony/tree/master/src/Symfony/Component/HttpFoundation
- If using the full stack, how it's replaced by it: https://github.com/symfony/symfony/blob/master/composer.json#L40