When does a modular monolith stop being enough?

We run one application container and one PostgreSQL database, and it has been fine well past the point people warned us about.

The honest answer seems to be: split when a single component has a genuinely different scaling or availability profile, not when the repository feels large.

What forced the first real split for you?

Ours was the live event stream. Everything else was happy on one node; fan-out was the one thing that needed a shared transport.