I agree, and what is more, I say, it is a grammar already, if a simple one.  You declare what you accept and what's to be done, making it a DSL expressed as an array of structs.

The only thing it lacks is that old getopt is a bag on the side rather than being integrated: struct option should have an additional member "char short_option", where '\0' means "no short option".  Given that feature and three per-program values "progname" (argv[0] by default), "version", and "usage_string", the --version and --help options can be processed inside getopt itself.  I especially like that you pass per-option pointers saying where to put the value, so no case statement required, just create some global or local variables and pass in their addresses.  Automatic support for "--nofoo" given "--foo" would be good as well.