Now it is for orthodoxy (or you) to demonstrate that these are special cases and that languages can in fact change in amazingly short periods. Unfortunately the only cases they do cite of this phenomenon is a) Anglo-Saxon turning into English in about 300 years and b) Latin turning into Italian, French, Spanish, Portuguese etc in abut the same time-frame.
Equally unfortunately, and for some reason orthodoxy never makes clear, both these 300-year spans are precisely when the continuous record breaks down.
So we have the fascinating case that, according to orthodoxy
a) Ango-Saxon/English has a continuously recorded history for the whole period c 600 AD to 2000 AD except for the crucial three hundred hiatus
b) Latin/French has a continuously recorded history for the whole period c500 BC to 2000 AD except for the crucal three hundred year hiatus.
Even you must smell something fishy there. Actually I don't mean "even" you but I appreciate it is a big step.