The language went semi-extinct for a while; there was a period with no living speakers, and Cornish as spoken now was reconstructed from texts. Which means that we don’t know exactly how it used to sound, and the speakers of the older Cornish would probably not understand speakers of reconstructed Cornish. (Which sounds profound, though speakers of 17th-century English would have similar problems with modern English.)

Until the revival in Celtic nationalism from the late 19th century, people assumed that the Cornish consider themselves as indisputably English as those from historic kingdoms like Mercia and Wessex (and that the Welsh, once they see reason and give up their language, will do so as well, and in due time, there’s hope for the Scots and the Irish); the idea of a modern Cornish national identity would have sounded absurd to them.