You could, but the incompatibility is not necessarily worth it.
There was some talk to take advantage of the transition to modules to be able to mark translation units as implementing a specific version of the standard (I think rust doese something similar) to allow for backward incompatibile Evolution of the language.
The committee doesn't seem to keen because they fear the language fragmenting and from a more practical point of views we will be still #including legacy code into new modules for at leas a decade (and I'm probably wildly optimistic).
With the glacial pace in which new language features are picked up by users of C++, I'd be surprised if modules have significant adoption in the first decade after C++20. It'll probably take at least two to three years to get stable support in most of the tooling and then anther couple of years until people start to believe that they are battle tested enough.
There was some talk to take advantage of the transition to modules to be able to mark translation units as implementing a specific version of the standard (I think rust doese something similar) to allow for backward incompatibile Evolution of the language.
The committee doesn't seem to keen because they fear the language fragmenting and from a more practical point of views we will be still #including legacy code into new modules for at leas a decade (and I'm probably wildly optimistic).