With WordPress, there are 5 different plugins already built that have more features than the dashboard you're planning to build, and the agency/person maintaining the site may already be familiar with them. WordPress and other PHP CMSes may not have the best architectures (Drupal was downright atrocious), but the ecosystem is thriving with pre-built, customizable tools for the 99% of customer needs.
Virtually all web hosts, including very cheap ones support PHP-cgi and MySQL, so deploying WordPress is frictionless. CMSes written in other languages have deployment more friction.
You find 4-5 real people that need a Wordpress site every month? Where do you find these people?
Designers and agencies are more than happy to continue to use it, and frankly they should -- it is their bread and butter. The WP drama is news for us web-devs but will affect their market in no way whatsoever.