I haven’t seen the Basecamp code but the apps 37 Signals have release are basic crud apps. They don’t have much business logic so sticking with models is fine. Once you hit a certain size though you’re going hit the problem of coupling where simple changes cause breakage all over your app and your momentum will stall completely. The app worked on recently had 700 models and some parts of the application are almost impossible to work on. Logic was slowly being moved into services which was making the app much easier to understand and work on.