I figured out how Google can solve syncing SMS messages. If someone's phone gets an SMS, Google should recognize it and mirror that text. That way, the text can be displayed on the phone, tablet, computer. You should then be able to send a message from your computer, where you don't have SMS, and Google should then send the message as an SMS ala Google Voice.
That would be clutch and a perfect solution. You wouldn't ever deal with SMS personally. Google would send it through their servers, like they do Google Voice already.
The only catch is phone numbers. But assign every Hangout user a phone number that links up with their email address. Just use email addresses as contacts.
Or something. I'm not getting paid here. I came up with this brilliant halfway solution. Let the engineers figure out the rest.