For Rust programs there's https://docs.rs/clap_mangen/0.2.31/clap_mangen/ that will generate a man page out of the help. (I am sure most programming languages have something like this). However, that's only useful if you are compiling the program (maybe distros could patch Rust programs to generate the man page during the build process)

A more general tool would be pretty good. Either for distros to call during build, after building the program proper; or for users to call.

If users are calling directly, it would be useful to, by default, show the regular man page if it exists, and only if it doesn't exist generate and display one out of --help. Also give it the same flags as man etc. In this case, we could do alias man=better_man and pretend this problem is already solved (but it's still better if distros generate it, so that they can display the man page on the web, etc)