I do believe that Hebrew was more fluent than what some think, since there are lots of Hebrew idioms in the NT, and the Dead Sea Scrolls are an abundant source of Hebrew manuscripts that cover the period of 300BC to 100AD.
The original languages were not lost, but they were used by those who actually had a purpose for them. There were some Christians who had possession of Hebrew Scriptures, even New Testament, as St. Jerome has recorded. Since the use of Hebrew was very isolated then the demand for Hebrew manuscripts were lower than Greek or Latin. Fortunately we have had some Church Fathers and early Christian writers who was skilled in Hebrew, such as Jerome and Origen, some say Tertullian knew it, which I don’t doubt because he was brilliant.