How reasonable is the idea of using older predecessors of modern language families as IALs? What advantages/disadvantages would this have compared to engineered IALs? Many people have tried to create auxiliary languages to ease communication between different speakers, i.e. Esperanto, Ido, Interslavic, etc. My question is, how reasonable is it to simply use an older language in place of a designed IAL; for instance, using the hypothetical Proto-West-Germanic language to communicate between speakers of West Germanic languages, or Proto-Austronesian for its speakers. What is the feasibility of this idea (in terms of pros and cons, not in terms of real-world implementation)?