Here's a thought experiment: Suppose that Mongo or Cockroach consists of 80% code for which they own the copyright (written by employees). And it's 20% written by contributors who have NOT signed the CLA.
Now can they relicence the whole codebase? No, but I think they can do something with thes same effect by relicensing their parts.
I think it's easier to see if they start a new project. Say Mongo starts a new project called "Dumbo".
Dumbo consists of 80% Mongo code relicensed. And they simply reuse the contributors' code under the existing open source license. So you distribute both licenses with the code.
But now it is still impossible (*) for Amazon or whoever to stand up a cloud service according to the license -- unless they want to rewrite 80% of the code.
I'm not a lawyer but that's my understanding of how it works. Interested in contrary (informed) opinions.
(*) edit: better to say that it's harder, not impossible. They can fork the old code under the old license. It really depends if the they "understand" the code.