> if they won't make it a paid service with some additional features, that means it's a cost center that could go away at any time

Project Fi is a paid version of Google Voice with additional features. I doubt Voice will go away before Fi does.

This right here is the quicksand you stand on with Google's free products. Starts out pretty innocuous. Hey, check out this cool new free phone product called Google Voice. You check it out, it's cool, don't really know how it might be useful but you play around with it and maybe use it for a side project or as a toy. Eventually, maybe years later, you find it may be very useful to your business. Great! You roll it out, slowly more and more things depend on it as you incorporate it more deeply into your services. Then suddenly, like a turkey on Thanksgiving Day, you're permanently cut off from your account without warning and without recourse. Now you're in a state worse off than had you never signed up for the service to begin with.

Google Voice numbers can be ported to Project-Fi. Porting to another carrier would almost certainly be more successful from that service.