I don't think everyone assumes that at all. Most discussions on DCI have been about "in big applications, this would work better" instead of "you should do this

I don't think everyone assumes that at all. Most discussions on DCI have been about "in big applications, this would work better" instead of "you should do this every time". Some of the supposed benefits aren't even seen in smaller chunks of code. Discussions on modules and SRP have not been absolutist either. People have said things akin to your last paragraph: for simple projects there's nothing really wrong with the normal approach. The problem starts when it grows. The whole "movement" of Rails refactoring only began after people started having trouble moving large codebases around. The guidance thing would be interesting. A collection of situations when things start to go weary, code smells. The thing is you can't do "when this happens, do this" because the "do this" part is the many solutions people have proposed. However I think people have those code smells a bit down already and generally know what a controller should look like. The placement and structure of the domain logic is what is bugging.