The catch is the tooling. If you have the time and resources to make the tooling that is necessary to make it work specifically for your org, than great! But if you don't, then a monorepo will generally slow you down because it will require coordinating changes across a much bigger group of people. Monorepos are great for very small companies with a low communication overhead, and very large companies with the resour…
It will slow you down because you're forced to confront the consequences of your change in real time, not down the road when someone notices something changed somewhere else that may or may not have broken their invariants in ways they now have to figure out. Tooling isn't the issue here, you're basically arguing for kicking the can down the road and encouraging a buildup of technical debt.
When you write code, do you put everything into a single function, or do you use separate functions for different parts of the code? Multi-repo is just the extension of that.
Software quality is orthogonal to your development methods. You can have good quality multi-repos or bad ones, and you can have good quality mono-repos, or bad ones.