why not abandon controllers at this point and route directly to show_thing?

Route directly how? Write my own HTTP layer? It's so easy to take advantage of Rails' routing and view rendering.

Well, instead of fighting rails, you could just use the lower level stuff it is based on which does the bits you want:

Routing - https://github.com/rack/rack

Templates - http://www.ruby-doc.org/stdlib-2.1.1/libdoc/erb/rdoc/ERB.htm...

Rails adds quite a lot of overhead and complexity, not to mention memory usage, so if you're going to bypass most of it, it might be better to start with something more bare-bones?