Make the argument that a rewrite in MVC would be an investment in a more extendable and flexible system, i.e. a plugin system could be created to allow the same

Make the argument that a rewrite in MVC would be an investment in a more extendable and flexible system, i.e. a plugin system could be created to allow the same CMS to provide more bespoke features such as a readily available blog or shop while being much more maintainable. A solid base to build on while saving money in the long run, this is something they could jump on board with. Or Make the security argument, a procedural system the size of CMS and site running on it ->could<- be riddled with security issues especially as developers are acknowledging that any addition is a bodged job due to global variable etc etc. If they are dealing with larger clients this should concern them enough. Also if you're lucky enough to be deploying to PHP 5.3 using closures (http://php.net/manual/en/functions.anonymous.php http://php.net/manual/en/functions.anonymous.php) can save a lot of hassle when dealing with procedural code.