> Escaping rules are well-defined

Except when they aren't? I'm thinking of things like the bash exported function bug, an obscure feature nobody thought was there turns evaluating environment variables into RCE. Not to mention different shells may do things slightly differently and you may get caught doing something unexpected.

So why not leave the shell out of the picture entirely? I think execve and friends are much easier to get right than escaping. There are still possible security bugs to consider (eg., if using the -p family such as execvp, who controls PATH? Is anything on PATH writeable?), but the shell won't be among them.