> If your testing is good enough, I don't see what the issue is?

Your testing isn't good enough. I don't know who you are, what you are working on, or how much testing you do, but I will state with confidence it isn't good enough.

It might be acceptable for your current needs, but you will have bugs that escape testing - often intentional as you can't stop forever to fix all known bugs. In turn that means if anything changes in your current needs you will run into issues.

> We release everything weekly, and some things much more frequently.

This is a negative to users. When you think you will release again next so who cares about bugs it means your users see more bugs. Sure it is nice that you don't have to break open years old code anymore, but if the new stuff doesn't have anything the user wants is this really a good thing?

No offence, but you might be a little confused by how complex your actual delivery is. That sounds simple. That sounds like it has a clear roadmap. When you don’t, and you have very agile development that pivots quickly and demands a lot of change concurrently for releases that have very different goals, it is not possible to make all your ducks sit in a row. Monorepos suck in that situation. The dependency graph is so complex it will make your head hurt. And all the streams need to converge in to the main dev branch at some point, which causes huge bottlenecks.