if you have a Yosemite mac, and enable SMS forwarding, the SMS and iMessage will automatically go in the Messages app in the Mac. you can therefore archive these messages in the app. And even if the Mac crashes/dies, the Time machine backup will have a copy of the SMS+iMessages, forever.
Also, The SMS and iMessage are automatically saved in the iTunes/iCloud Backup, however, this has 2 drawbacks:
- If the iPhone is lost/dies, you'll be able to recover the messages only by putting the backup on a newly reset/bought iPhone.
- the backup only saves current SMS/iMessage, which means that if you manually delete from the iPhone the messages, it will also delete from the backup next time the backup syncs. This therefore doesn't allow "archiving".
So the first solution is best if you have a Mac, especially since it's for legal reasons, for which it is recommended to have 2 versions of the files.
Hope this helps 🙂
<Edit>: Also, you'll probably never find an app that syncs messages, since iOS is a sandbox OS, meaning the Apps don't have access to the data of other apps. so Messages data isn't available to any other app.