I suspect a lot of people become expert in a "CMS" like WordPress or Drupal, because it seems easier than learning to work with a proper framework like Rails, and then find themselves stuck inside the platform.
Like project management systems, CMS is destined to be reinvented by everyone, every day of the week.
It's very hard to convince people of going with a proprietary, custom solution. Many people have been burnt here, and it's a tough sell to upper management.
All CMS is necessarily a "proprietary, custom solution". All CMS (like all software) exists somewhere on the spectrum of "easy to use" vs "extensible and customizable".
If management chooses to rely on a vendor to set the roadmap, then they are fools, and there's not much you can do to help foolish management.
Like project management systems, CMS is destined to be reinvented by everyone, every day of the week.