An existing safety mechanism is to do exactly what Linux does: have a copyleft licence and no CLA. So that the copyright is shared between the contributors (so it's impossible to change the licence) and the licence enforces sharing your changes of the project.
Previous versions are still available under the original license, right? So if you don't want to use it with a new license, you're in the same situation as if company went out of business or stopped support and development for any other reason. There are no safety mechanisms for that either.