This seems like it should be a core function regardless.

I don’t see why that should be the case. If content is supposed to appear together, it should all be on the one page.

Gutenberg, the editor currently slated for 5.0, is more modular, but is still about page content all existing on the one page.

You could use a plugin like Advanced Custom Fields (you’d need the paid Pro version for this), to create a Repeatable Post Object field that you could then modify your template to pull in the content from the selected pages.