Old English is a fairly poor example because it was basically replaced by another language through conquest. While languages do evolve, it's a gradual process

Old English is a fairly poor example because it was basically replaced by another language through conquest. While languages do evolve, it's a gradual process and they remain understandable for centuries. Shakespeare, Dante and Goethe are quite understandable to speakers of modern English, Italian and German. A speaker of Spanish or Italian can learn to read Latin texts even now with relatively little study. When the language of Julius Caesar is separated from modern Italian by 2000 years, the change process was so gradual that culture of the Roman Empire is still very visible in modern societies. That is not comparable to, for example, Mesoamerian cultures that were wiped out in a shorter period of time.