Rails used as intended can be a real pain to upgrade, I agree.
Recently I have been developing applications in the "thin-model, thin-controller, with a service layer and external API wrappers" style, as espoused by Gary Bernhardt [1] and others [2] [3] [4] [5]. I upgraded to the latest Rails this morning on one app, and apart from swapping the Money gem to use the git repository, until they have a new version out with compatible dependency, it went smoothly. All the specs and acceptance tests passing with no alteration required.