How should Google roll out SMS integrated Hangouts?

I've been thinking about this for a couple of days now. Is there really a way for Hangouts to integrate SMS in a way that becomes adopted and doesn't confuse lil Grandma with her Galaxy Ace at the same time?

Let's see what their options are:

1) Roll out an Opt-in setting that enables SMS in Hangouts through the Play Store

This wouldn't drive adoption of Hangouts very much as those who currently ignore Hangouts will continue to do so but would be nice for current Hangout users, I don't see this as a particularly appealing option.

2) Roll out an Opt-out setting that enables SMS in Hangouts through the Play Store

Remember this wouldn't be as simple as replacing Talk with Hangouts was. OEM's all use different Messaging apps with their phones and Google wouldn't be able to flip a switch and replace them all with Hangouts.

If this was to happy lil Grandma with her Galaxy Ace would suddenly have two apps on her phone offering to handle her SMS. She wouldn't be happy with that. Plus you'd need to convince her to swap her home-screen shortcut for Messaging to Hangouts which she may not know how to do, or more likely not care to do as she isn't interested in having Hangouts pushed onto her.

3) Remove Messaging and make Hangouts the default SMS App in 4.3.

This has problems we are all too used to with Android. The slow adoption of Android Updates. There is also no guarantee that OEM's and Carriers would be willing to just remove their Messaging apps and fully embrace Hangouts so adoption would likely be scattered and fragmented.

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Anybody have any other suggestions? With Android being in the situation/state it is in I cannot see it being possible for a roll out of a SMS integrating chat app as smoothly and elegantly as iMessages was with the iProducts and I'd suspect this is part of the reason it wasn't there at launch.