Blocking shares from outside your organization would be a fine solution for the many folks who just use Google Drive for personal storage or team coordination. Inter-organization communication isn't a primary use case the way it is with email.
But perhaps the way forward is, just as email pushed us ahead through massive interconnected communication, so is swapping files through massive interconnected communication. I'm sick of not being able to send digital artefacts > 25MB to people, and having corporate emails reject 15MB files.
Perhaps it's the next leap forward.
I think there's a good argument that's not the case, but I think there's an argument we should try it.
Allowing people to stop it, will kill the idea in it's tracks, so I understand why the Google Drive Team don't want to roll out a button. It's possibly a Google Drive killer, it's not what Google Drive is.
Any solution to this, should work equally well with email and vice versa.