With languages that have become "extinct" a major problem is vocabulary. As new technology, as an example, comes around new terms/words have to be borrowed and modified for the local phonemes etc or made up. There is no way to say "steam engine" in Sumerian, so you'll have to kludge together some words to make it up (like crunching German or Dutch words together), borrow the term and make it "Sumerian", or simply make new nouns out of whole cloth. As an example, this sort of thing was a difficulty when "modern" Hebrew was pushed in to general use as "religious" Hebrew never evolved terms for modern technology and concepts - words like "tank" and "transgender" are not in the Torah, Talmud, Mishnah, etc. The longer a language has been extinct, the more of this you have to do and soon becomes almost like making up a new language using grammatical and pronunciation rules from the old one. Of course, odes anyone really know what spoken Sumerian sounded like - of course not.