The good thing about google voice is that when it is finally axed, the US laws require them to give a way to port numbers. That's the only reason why I was comfortable using their service
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)
Or what? What's the penalty if they don't?