No, I don't agree with you. Things are broken up due to bandwidth constraints, and because navigational sophistication is limited by limited markup and interfaces. Anything larger is also unlikely to be hierarchical. This is incorrect.
Most big documents are richly, intensely, fundamentally hierarchical.
Parsing 300 pages will always be annoyingly slow. If document open time rises from one second to three, it's a big problem. 1.Existing SGML tools will be able to read and write XML data. A very large set of existing SGML tools can and do read XML documents. And a lot of them didn't need anything but a new SGML declaration.