I certainly hope they do not add an ORM into WordPress. It would add a big layer of complexity and break nearly all plugins that interact with the database.

This can be mitigated by maintaining and updating both the database access as it is today as well as the ORM layer.

Which would probably mean some plug-ins using one and some the other. Perhaps over a long enough time scale converging for the ORM.

Right now debugging problems with the interaction between database and application are blessed by it being fairly transparent

Adding a messy layer in between adds complexity not worth it unless it is required to add support for many other databases and I dont see any reason why it would be beneficial for enough users for that to be worth it.

Introducing just SQlite will expose a lot of problematic edge cases. since MySql provides a lot of feature that Sqlite does not. (which in most cases is a very good thing)