Not a big deal
Most people don't want to and won't use mobile VOIP, except for maybe international calls, if even that.
With the glutton of mobile minutes now available (unlimited anymobile, rollover, night and weekends, etc), there isn't much incentive to the customer to use anything like this, and carriers know that they can just force a voice plan with more minutes than anyone known what to do with in order to have a smartphone, so they will win no matter what.
The real issue at hand is that we have been trained to expect mobile data for $25 or $30/mo, which drives 95% of the network traffic, when voice and text together are $60/mo, which just makes no sense. With the mobile data surge, the companies need that kind of money to keep these massive nationwide networks running, but they are being dishonest about where the real costs are.
Yeah, it's useful for that, but an occasional international call doesn't fundamentally undermine the carrier's business model. You could also do basically the same thing with GV.