Exactly with this vision I started beginning of 2020 our Go monolithic backend. Currently, one year later with a few people developing it, it's working very well. And thank god I resisted and pushed back against people who were advocating to split off parts into AWS lambdas, misguided by confusing "physical" separation for logic separation.

And regarding the cost you mention, I don't think there is any. Yes, the components only know each other via their interfaces, not their concrete types. Yes, you need to define these interfaces. But that's in essence plain old school dependency injection. You'd do that anyway for testing, right?