Node can run multi-process with the Cluster module. Node being written as a monolith but running it in 100 instances is also an option. Roles can be implemented in software. There's much less "orchestrating complexity" when you're deploying a single service.

So Node being single-threaded is not itself a reason to use microservices.

I tend toward writing a monolith for the core API of a service, but then break out microservices for tasks that need to scale independently (or that need to run on high-memory/high-performance instances, for example). So I'm not totally against using microservices. But we should choose to use them when they're to our advantage to use them, not just "because they're already written that way."