One thing I heavily enjoy about monorepo's (I'm talking java/c#/c++ projects) is the ability to navigate the entire codebase from within an ide. That alone has caused me to migrate projects (medium projects ~20 developers) from poly to mono repos. Dropping tons of duplication in the build system in the process. I can think of good reasons to split projects along boundaries when it makes sense, but not blindly by default, and not without carefully considering the tradeoffs.