That would mean that it's impossible to mix old and new C++ codebases, which would make it very very hard to port larger projets. It may even make it impossible if one uses a header only 3rd party library.
It would certainly restrict the ways in which code could be mixed or updated. But I do not think that it would be as hard as you make it out to be. Let's say that you can have either modules without includes or more traditional translation units that use a preprocessor and can also import modules. Then you could port then code over one module at a time, couldn't you?
Not really. You can't wrap a translation unit that uses a 3rd party library into a module. That means every other translation unit that uses this unit also can't be a module and so forth.
