Default queue implementations come with co-operating queuing out of the box. I suppose you could use some hand-rolled queue with no internal locking, but this is disingenuous.

> you'll have race conditions or no cooperative queuing between multiple instances of the application.

You're thinking micro-service again here. There's no need to have multiple "instances" at all. Any concurrency is handled internal to the application itself with use of e.g. fibers, threads, etc. A good argument to why monoliths have drawbacks can't be "well its not a microservice". :)

> A good monolith still requires all the same hard decisions about modularity, high-availability etc.

Never said it didn't. What it doesn't require is all the tech stack to support that over many instances, because there's one instance. A large majority of µServices is just re-implementing what your runtime gives you for free.