Exactly, the author of the article apparently doesn't realise that if everyone switched to data-only usage, they would still be getting billed by the same carri

Exactly, the author of the article apparently doesn't realise that if everyone switched to data-only usage, they would still be getting billed by the same carriers, who would alter pricing so that the data fees bring them the required levels of revenue. Obviously this can definitely change in the future, whether it's a new type of technology, or Google funding wide-ranging WiFi networks, or some other option. But frankly that's a far bigger barrier than "but Google's VOIP services aren't as good as they could be".