not exactly the scenario but the fact remains the apple staff in applecare and supervisors there could not offer a better solution than this. ios4 to ios7 works however.
Correct me if I'm wrong, but wasn't iOS4 when it went from SMS to iMessage? That might explain the lack of support for importing from a 4-version-old data storage format.
Sure, it'd be nice if it would still import from the older stuff, but at some point maintaining the older code just becomes a maintenance hassle for very little return.