Why are entities still queried based on hook_schema()?

Problem

1. Trying to swap out an entity type implementation. (FWIW, User)
2. I can easily swap out the entity type's storage controller (entirely).
3. But the entire entity system and default DatabaseStorageController is still based on hook_schema() column definitions...?
4. I'm stuck in determining what actually needs to be swapped out and/or altered — but hook_schema_alter() totally feels like an alien in D8.

Proposed solution

1. Declare the database schema columns of an entity type in its storage controller (or entity info).
2. Remove entity info's dependency on hook_schema() (entirely).