I agree with much you say here - I have personally seen too many beginners jump to use Devise, Cancan etc (every app has to have authentication and authorization, right?) without first understanding the core of Rails, and it just adds unnecessary abstraction to what's already a heavy framework.

When we teach (we run an online web development bootcamp), we teach folks Ruby first, then the general web concepts (HTTP, HTML, etc), then lay the foundation of dynamic web programming (rendering templates, routing, etc) with Sinatra, before finally tackling Rails. When we do teach Rails, we stay very close to the framework itself and ask students to build things like authentication from scratch. After they get the core of Rails under their belt, that's when the fun really starts - we introduce comprehensive testing materials, advanced Rails topics, service integrations, complex application workflow, git/github based team collaboration and production app deployment and maintenance concerns.

It takes a while to build up all this from the ground up - but we found this being very effective in pushing people to an intermediate level and build up confidence along the way.