Base classes over interfaces?

[JeremyCaney]

In Section 4.3 of The Framework Design Guidelines, the authors make a compelling argument for using base classes over interfaces for cases where polymorphism or multi inheritance aren’t needed. This applies effectively to e.g., ITopicRepository (which already has a base class) and possibly the TopicMappingServices.

If so, should we consider changing this for OnTopic 6.x? This would be a major breaking change, but it would also afford us a more stable foundation to build off of in the future.