That seems like it'd get really awkward pretty quickly. "if" isn't unique in this regard; there are about a hundred shell builtins, and all of them can be used as an argument to a command. (For example, "echo then complete command while true history" is a valid shell command consisting entirely of names of builtins, and the only keyword in it is the leading "echo".)

You'd have to `|| EVERY_KEYWORD_IN_LANG`, and then if you ever add a keyword, now you're updating that list there, and anywhere else you've used it.

As the "Lexer hack" Wiki page says, this is only a problem if you're lexing in the first place. If you just parse the grammar, this isn't a problem.

The problem lies with shells extensive usage of barewords. If you could eliminate the requirement for any bareword to be treated as a string then parsing shell code would then become much simpler...but also few people would want to use it because nobody wants to write the following in their interactive shell:

    git "commit" "-am" "message"

    ls "-l"

 etc