Having worked extensively in the Shopify codebase, one of the largest rails apps in existence, I have experienced firsthand some of the downsides of working in monolithic codebases.
Keeping an application stable starts off being simple since your codebase is small, there are few points of failure, and you can easily reason about the implications of code changes.
True or False. Zero or One. Computers are viciously black and white in their logic. Humans, on the other hand, are messy — Emotional, forgetful, biased and opinionated.