I worked on a many years old and k*10⁵ LOC Ruby application, and it's fine.

Talking about 1 KLOC and future unmaintanability is nonsense.

In order to scale, Ruby applications need very high coverage. This has nothing to do with Ruby and/or Rails though; it's a requirement of all the dynamically typed languages.

There are certainly missing functionalities that at some point will cause a roadblock (currently, threading is immature), however again, no framework can support all the possible functionalities.