I don't know if this is solvable. A CMS is complex enough that each of its functionalities deserves the care of good product and technical designers. These pieces ought to then be integrated into a cohesive whole, a la carte. For desktop software, we were able to pull this off pretty well, with 2000s OS X as the most cohesive, successful attempt, dictated by strong design guidelines and solid enough tech... And note that they invisibly transitioned CPU architecture along the way! But when online collaboration and multi device access became a requirement, everyone flailed and forgot the lessons of the past.
I don't think the current software market is capable of something like that. Almost everyone is trying to make captive SaaS where compatibility only exists on a service-to-service basis, in a subordinate manner. Branding is more important than cohesion, and flexibility and interoperability has taken a back seat to dumbing down.