Hangouts Idea: Why not make an API?

I've thought about it lately, and probably like me, many of you want to use Hangouts but can't because none of your friends use it. Now the reason iMessage works is that you don't need to sign up - everyone with an iPhone can just use it, and they'll probably won't even notice. A lot of my friends with iPhones actually think they're still using SMS.

Now you might say Hangouts is installed almost on every phone, but it's still not the stock app. Most people just ignore it and use the default SMS app, just because it's the default app.

So why not Google just work with the OEMs and make a Hangouts API, so the stock SMS apps can also serve as Hangouts apps. That way people would still use the platform, the way they use iMessage.

Now there is a branding issue here, but in the current state of lack of users, isn't that an easy solution?