I would agree but GV seems to be a forgotten product and I'm not going to tie something as important as my phone number to something Google might shut down on a whim.
Google has recently integrated Google Voice into Hangouts. They probably won't update and may deprecate the Google Voice site and apps. They have also been pushing phone calls from Hangouts. My suspicion is that Google will open up the usage of virtual phone numbers in Hangouts.
I don't think that's going to happen. GV has been around since 2009 - I got my number then and it's been great. This is not Reader - several million people and businesses have GV numbers. Of course it can happen, and there may be other reasons you might not want Google to handle your phone calls, but I estimate the probability of Google just shutting this down as pretty small.
At worst you'll have to transfer your number to some other service. While I wouldn't be shocked by Google shutting down Voice, I would be shocked if it just abruptly vanished one day with no warning.