It’s about avoiding single points of failure.

> In the world you describe, you don’t have any durability when the network is impaired.

Yes, the real world. If you want durability, a single physical machine is never enough.

This is standard distributed computing, and we’ve had all (most) of the literature and understanding of this since the 70’s. It’s complicated, and painful to get right, which is why people normally default to a DB (or cloud managed service).

The reason this matters for this logging scenario is that I normally don’t care if I lose a bit of logging in a catastrophic failure case. It’s not ideal, but I’m trading RPO for performance. However, when regs say “thou shalt not lose thy data”, I move the other way. Which is why the streams are separate. It does impose an architectural design constraint because audit can’t be treated as a subset of logs.

> If you want durability, a single physical machine is never enough.

It absolutely can be. Perhaps you are unfamiliar with modern cloud block storage, or RAID backed by NVRAM? Both have durability far above and beyond a single physical disk. On AWS, for example, ec2 Block Express offers 99.999% durability. Alternatively, you can, of course, build your own RAID 1 volumes atop ordinary gp3 volumes if you like to design for similar loss probabilities.

Again, auditors do not care -- a fact you admitted yourself! They care about whether you took reasonable steps to ensure correctness and availability when needed. That is all.

> when regs say “thou shalt not lose thy data”, I move the other way. Which is why the streams are separate. It does impose an architectural design constraint because audit can’t be treated as a subset of logs.

There's no conflict between treating audit logs as logs -- which they are -- with having separate delivery streams and treatment for different retention and durability policies. Regardless of how you manage them, it doesn't change their fundamental nature. Don't confuse the nature of logs with the level of durability you want to achieve with them. They're orthogonal matters.