Most DMS platforms like Documentum and SharePoint have their out-of-the-box full clients which are meant to provide all (or most of all) of the platform functionality. Although these full clients can, normally, get customized or configured, they are, however, complex and slow to adopt new technologies.
If you have a specific use case, why start from a complex system and reduce it to your needs? Why not build a custom client that is streamlined and focus on your use case instead? The business logic can reside on the server. But for the client, we are free to use any state-of-the-art modern web framework.
I’ve been in the Document Management System (DMS) / Enterprise Content Management System (ECMS) market for more than 20 years. Sometimes very focused on a specific aspect e.g. Technical Documentation, sometimes more general e.g. ECMS platform and sometimes with focus on an industry segment e.g. Life Science. I have seen a lot of vendors, products and technologies coming and going. The latest acquisition and certainly the biggest one was just a week ago. Hopefully, this will not reduce the power of innovation.