Imho, with mono repos you can achieve all the benefits of a monolith with the benefits of a micro service architecture.
Except you're now calling functions across the network. And no more complete tracebacks. And need to deploy 10 things at the same time or everything falls apart.
Yes, you get the benefits, but also all the drawbacks. Those drawbacks might make sense for extremely large teams. I want to cry every time I see a 20 engineers teams wasting company's money with this.
Why build libraries and separate services if an update on some part of the codebase needs changes across all the platform?
This trend of microservices + monorepos sound to me like taking the worst part of everything.
If you do microservices, then let each team have their own stack, their own tools, their own libraries and be independent and just agree on the APIs.
If you want everything super consistent and share as much code as possible, etc then just do a traditional (well architected) monolith, maybe deploy it with different configurations for different scaling needs, etc.
Monorepos are to me a symptom of worse problems.
I worked for a.company where they had a rule that "every repository should be a monorepo". They didn't even understand everything that was wrong with that rule.