Agree with your sentiments, I feel mostly held back at this point with my own inexperience at building the right context 
I think most of the wins will come when the app already has overall pretty good patterns, and then codifying those rules in a rule corpus for the agent. You could then use something like Claude Code GitHub Actions - Claude Code Docs to provide users feedback on how to handle violations, in a way that was previously impossible to automate besides general rules.
On this topic, I’ve been thinking about the reverse situation too where agents could deliver value into codebases without an established “Big Rails” architecture. For example, take the idea of a maturity model from this post. Maybe it would be possible to build a packaged context that you could add and use to have an agent guide you into incrementally establishing modularization starting with a low maturity. Step 1 would see the agent guide you through the necessary discussions to determine domains and dependencies, help you set up project structure etc etc
Similar to this but more geared towards Ruby at scale\Packwerk