I think the emphasis is on official. That is, it would function as the common language of administration, communication, diplomacy, etc. (i.e., lingua franca), but it wouldn't replace vernacular languages. This was the norm centuries ago in Europe.

One advantage of it being "dead" is that the meanings of terms are much more stable. They don't undergo the usual slippage and mutation of spoken languages. This advantage would be lost if it were to replace existing vernacular languages.