One document == one HTML page was never the idea. Documents are often way too long to comfortably read and navigate that way. Breaking them into sections and linking between them was part of the core idea of HTML.

Includes are a standard part of many document systems. Headers and footers are a perfect example - if I update a document I certainly don't want to update the document revision number on every single page! It also allows you to add navigation between documents in a way that is easy to maintain.

LaTeX can do it. Microsoft Word can do it (in a typically horrible Microsoftian way). Why not HTML?