What about all classes of durability errors that do not manifest in corrupted checksums (e.g. "cleanly" lost appends of entire transactions to the WAL)?

It seems like you're focusing on a very specific failure mode here.

Also, what if the data corruption error happens during the write to the actual database file (i.e. at WAL checkpointing time)? That's still 50% of all your writes, and there's no checksum there!