It is a massive scaling issue. Most enterprise JVM apps for example are running with 8GB+ of RAM. Adding 10x the capacity which is only needed for say sending emails can be hugely expensive or sometimes impossible if you are running on-premise.
And scaling different parts of the application is just one of many benefits of microservices.
I would think that the point of writing a single process monolith with components communicating via a bus like this is that it’s a small step to break out one of those components into a separate process and scale it horizontally.
And scaling different parts of the application is just one of many benefits of microservices.