If we look closer, I believe (not an expert, though) that without a copyright assignment your contribution would have to be kept with the same license. We'd be just getting into the fact that non-copyleft licenses typically allow to use the code for building proprietary software. But AFAIK the contributed code itself would need to be kept with the original license? (that's where my knowledge is lacking)
There’s no devil in the details. The entire codebase can be taken proprietary right now and there’s not a thing you can do. You’re free to extend the last permissively licensed version.
OTOH if I had signed a CLA, then your company would be within its right to entirely strip any attribution to give whatever license they see fit to the code I wrote.
Some of us prefer permissive licenses, or choose between permissive and copyleft depending on the nature of the application.
But here's something you can't do with my MIT licensed code: change the license to copyleft.
Point being, if one is contributing to a project with a permissive license, that should imply being a-ok with it getting rolled into some binary in a form you don't have access to.
If one is contributing to a project with a permissive license, that should also imply being a-ok with it getting rolled into a copyleft project too.
Examples of permissively licensed code being used in proprietary systems abound.
Has anyone ever successfully copyleft-jacked such a project? I grant that it's possible but I'm not aware of it actually happening.
That aside, any permissively licensed library used in a GPL program would technically fall under this situation, as it ends up distributed under the terms of the GPL. It just so happens that usually the libraries aren't modified by the application developer, so you can typically go find the original source.
If they are, those changes may be under the GPL, depending on whether the application developer kept the derivative version of the library explicitly under its original license, or licensed their changes under the application's license.