Earlier quoted context omitted.
It seems like someone did do them! Obvious examples include: 1. basecamp/fogbugz/jira 2. Confluence/Sharepoint 3. django/rails 4. Google Docs 5. Facebook
I think there is a lot further someone could take #3 (crud apps for business). While django/rails are great, I think you could build some more specific tools that would provide a lot more leverage in some cases. (Maybe something like DabbleDB but with some more specifics built in.)
Both frameworks support plugins you know.