You generally can't throw a rock in a crowded room of programming nerds and not hit someone who has built their own CMS these days. I've basically built my freelance career out of building and selling a set of CMS software and I'm sure others here could say the same.

For a simple term like Content Management System, it seems like everyone has a different interpretation of what that means. I think that alone answers you question about whether or not this is a solved problem. There's always going to be some target market with a need that can't be solved with current CMS software.