How does writing an application as a single language/binary prevent read-replicas or redundancy?
The application could do that.
Or you could do it at disk level (RAID).
How do you manage multiple instances of the application and not introduce split brain?
> Or you could do it at disk level (RAID).
That is not comparable. RAID's redundancy is not the same as the redundancy in a multi node database cluster. You have one service, not multiple services, network card gets fried, your database goes down, you can't promote a standby to master and be on your way. Also RAID is a single disk as far as the OS is concerned, so you could hit I/O limits (especially if you have a single binary) that cause your app to chug, you cannot split your writes and reads across different physical or virtual machines that have different disks.