Give people your Google Number. If Google changes things, they said for some fee, like $25 you can port away. Although other VOIP services usually charge to port TO them, I would bet that most of them would waive that fee to get the mass of GoogleVoice customers who would jump ship.
You must know that whether a phone number or email address remains forever is not that big a deal. With a search engine or call forwarding, etc, it is easy enough to change business cards and stationary.
Delta Airlines used to not own
Delta.Com. In fact, they started out on the web as
DeltaAir.Com. Imagine their cost to change a domain name.
I guess also, that if $6 a month is a deciding factor for you as to which service you choose, perhaps pay the $6 and be done. I would not use GoogleVoice for a mission critical (ie business use) if the changeove to someone else it that much of a worry for you.