I have worked on a decent number of dynamically-typed Ruby and Python systems in the 50,000 to 250,000 line range, written and maintained by teams. This has never felt like a strong use case for dynamic typing. You end up losing:

- A lot of IDE support. The loss of documentation tooltips, in particular, can be painful in a team environment.

- The ability to change an API and immediately see all the affected code. This affects refactoring speed when making big cleanups. Massive updates I could do in a few hours in Rust might take 2 weeks on a big Rails project.

- Team-wide clarity on exactly what goes into key data structures. Can something be null? Does it allow numbers, or only strings? Etc.

With two developers and a small code base, you can keep most of this information in your head. And Rails is still unmatched for terse, clear code, plus off-the-shelf modules for many common tasks.

I'm not even sure that Ruby could be retrofitted with a really worthwhile type system, to be honest. JavaScript already required a lot of black magic, and in some ways, Ruby is even more dynamic. So perhaps Ruby is better left as-is, even if that makes it a poorer choice for projects that would benefit from static typing.