Your designer is right and wrong. They’re right that it would take a lot of work to recreate the example website in any WordPress theme, especially since the example is all flash. But, you can use Enfold or any other theme to create an HTML alternative to your all flash example. I agree with Torlock that you could create a more extensive and better website using Enfold or any of the more popular WordPress themes.

I believe your designer is wrong with their comments about ruining optimizations. With the use of child themes you can make changes to appearance, etc. and continue to keep these settings while updating the theme core. WordPress actually recommends creating a child theme, and many designers (e.g. Kriesi) have child themes as part of their theme files.

There are security vulnerabilities in most if not all programs. So as developers find issues with their software they close the hole, therefore it is essential to keep your software up to date. Furthermore, hackers love going after php code and they look for security holes. Therefore, it is important to keep your programs up to date.

Just like any piece of software developers are constantly adding functionality to their software to make it easier to use, more powerful, etc. so you need to keep they software up to date. WordPress, Joomla and any other CMS is no different. So they have to be updated on a regular basis.