Also consider some practical concerns with the command structure you describe: Tab completion of commands wouldn't work anymore, unless you supply custom tab completion setups. The direct association between a command and its man page would be broken. Shell scripting becomes more challenging: Instead of writing common things like "if which pg_waldump; then" you'd need some custom code, to be determined. These are all solvable, but just a sum of slight annoyances, for no real benefit.
I don’t think the man page concern is justified. We could have a “help” subcommand, just like git; “git help add” is (to a casual observer; probably not precisely) the same as “man git-add”.