I think it's going to be too much pain to try to transition the
:active_record load hook from executing against Base to executing
against Model.

For example, after Model is included in Base, and modules included in
Model will no longer get added to the ancestors of Base.

So plugins which wish to be compatible with both Model and Base should
use the :active_record_model load hook which executes *before* Base gets
loaded.

In general, ActiveRecord::Model is an advanced feature at the moment and
probably most people will continue to inherit from ActiveRecord::Base
for the time being.