"A societies religions tend to "kill" the language that they have reserved for Sacred/sacred worship. That's just a fact of history."
Didn't happen in Greece, though demotic is somewhat different from the Byzantine Greek of the Divine Liturgy or the Koine of the NT. It is understandable by most Greek speaking people.
A language doesn't have to be incomprehensible to be considered "dead" or reserved for worship.
The Anglicans railed when there were talks of updating the Elizabethan English which no one speaks even though it is still understandable with some effort.
Koine and Modern Greek are more extreme examples of this same principle.