It's not really a red herring when Lambda mostly forces communication between different application layers to be over the network. You suddenly have to deal with rate limiting, retries, bulkheading, etc. in a lot more places than you would even with even a SOA.
> you actually have to write unit tests
I'm not really sure unit tests are a differing factor in testing functions as a service and other software. The real issue with testing is that it's very difficult if not impossible to do more than unit testing locally. You're often using docker containers or mock services to try and simulate what is going on in production which is insufficient. Things like CloudFormation can't be done locally (and this can be where a lot of gross complexity lies), so ultimately you have to set up an identical-to-prod testing environment and do the bulk of your testing there (which you probably should have anyway, but that's besides the point). Things like localstack HAVING to exist but not being supplied by AWS is a symptom of something gone wrong.
> Would it be simpler to write a full-blown service that polls events and listens to message queues and launches background tasks? Obviously, no.
Why is this obviously no? It isn't to me. You likely need a lot less these type of things if you're running a monolith.
> In terms of complexity, they pale in comparison with even the tiniest hello world windows GUI app, or even Qt.
It seems to me you're ignoring all the extra stuff you HAVE to have to deploy, secure, and maintain serverless. There is a lot more than the code for the function themselves.
