In Romania WhatsApp is fairly popular, but it's declining. About 60% of my phone's contacts have had it installed at some point, but I noticed that many of them have uninstalled it.
Its popularity has been declining due to Facebook and Android - Facebook on Android works really well lately, most of my acquaintances are on Facebook already and everybody with an Android (which is most of my acquaintances) has either Facebook or Facebook Messenger on it. This is happening because Facebook is the messenger that finally dethroned Yahoo Messenger in my country - really, I'm serious, Yahoo Messenger was still really popular until about 2 years ago. And Android penetration has been phenomenal, due to cheap models being advertised by mobile carriers and sold instead of feature phones.
> Their logical endgame is to move all messaging to the Hangouts protocol, and fall back to Google Voice to deliver messages to users who don't have a Hangouts client.
You're a US citizen and so you might have missed the part where Google Voice is not available outside the US - no alternative can win over SMS, unless it is international.
I actually hate them for trying to take over my SMS messaging and I'm glad that I still have the option of not allowing them to do it on my Android, even though I'm also a regular Hangouts user. What is it with these companies trying to be everything to everybody?