The one thing that falls over is that the webhooks are actually self modifying code since you have an 'on commit' webhook that bootstraps the actions logic from the workflow.yaml file(s) (which are not really config files, they have logic that needs to be evaluated).

I don't disagree that it's obvious they've got problems but I'm just saying it's obvious to me the part that falls over (the scheduling of jobs) and why that would impact self hosted runners, which do no scheduling but depend on it to function.

As for 'just a message queue with some database updates and sharding that's easy to reason about'... Here's a job scheduling problem as an example: imagine you schedule a job, and there's no runner available. How do you disambiguate between no runners available because you've reached capacity, runners not being available because they're on a real network with faulty connections, and runners not being available because of a faulty rollout of internal updates?

A simple message queue for job scheduling is fine if you own everything and can deal with the operational overhead of identifying those cases by hand, but Github can't do that.

It's just silly to guess at how a system works, or should work, without digging into the details. Because you simply don't know what you don't know.