The C++ committee tries to do features with room for future extension. They believe that whatever you want from sub-modules is still possible in the future - but better to have a small (as if modules is small) thing now than try for perfects. We can argue about submodules once we have the easy cases working and hopefully better understand the actual limitations.

[delayed]

Not to put too fine a point on it: The world has 35 years of experience with submodules. It's not rocket science. The committee just did what committees do.

And sure, "future extension" is nice. But not if the future arrives at an absolutely glacial pace and is technically more like the past.

This may be inevitable given the wide spread of the language, but it's also what's dooming the language to be the next COBOL. (On the upside, that means C++ folks can write themselves a yacht in retirement ;)