Log files have a long life, potentially in order of decades, so that sets the level of backwards compatibility required. It is huge, and indicates that whatever the merits of not documenting the format, it will become set in stone anyway unless log conversion tools are provided which can perform the conversion and afterwards validate that every bit of the information is old version was preserved and correctly converted (which might be same as checking the hash value of the log entries).
Nevertheless, even if archived logs become unreadable, old versions of this software do not just vanish into the ether but remain runnable, at the limit through emulation of x86 instruction set and old linux kernel versions. So some solution will always exist.
Regardless, I'd say that the reasonable requirement is that every generated journald log file must remain readable forever, or a chain of provably non-lossy converters must be provided that can upgrade from the earliest version.