Journals do not protect the filesystem metadata from corruption. They
protect it from being in an inconsistent state after a crash (i.e. part of
the metadata written, other parts not written).
If a kernel bug, cosmic-ray-induced bitflip, or transient drive bug
corrupts the filesystem you still need a fsck. And in the end, that *will*
happen, even with PCIe and ECCRAM: after all, the *CPU* doesn't checksum
everything inside itself, and ECCRAM can't detect all possible failure