This will be great until Google sunsets Google Voice next year.
Or what? What's the penalty if they don't?
You can initiate the request from the other side (the new carrier), in which case it is forcefully wrested in a day. If google still refuses, you can complain to the FCC.
As for google penalties for holding your number hostage, given that the business is dying there's really nothing other than an FCC lawsuit (which, as you imagine, would be for a paltry sum)
If I have my way, sooner or later it will be more convenient to just run your own Google Voice clone on Twilio, Tropo, etc.