They also have the guarantee that the code licensed under the GPL, and all future enhancements to it, will remain free software. The same is not true of the MIT license's weak-copyleft.
Neither GPL nor MIT (or anything else) protects you against this.
(EDIT) scenario: I make a browser extension and release v1 under GPL, it becomes popular and I sell it to an adtech company. They can do whatever they want with v2.