But first I make a protestation
That I am drunk; I know it by my sound
And therefore, if that I misspeak or say,
Wit it the ale of Southwark, I you pray
For I will tell a legend and a life
Both of a carpenter and of his wife
The spelling isn't even that different (though the pronunciation is more so); compare the fifteenth-century spelling from www.chaucermss.org (multitext edition):
But first I make a protestacioun
That I am dronke / I knowe it by my sown
And therfore / if รพ I mysspeke / or seye
Wite it / the ale of Southwerk I preye
For I wol telle a legende and a lyf
Bothe of a Carpenter / and of his wyf
(The slashes appear in the fifteenth-century text; I'm not sure what they indicate.)
Modern English and Middle English aren't quite mutually intelligible, but if you were somehow dropped into England in 1420, you'd probably be conversational inside of a month.
Mutual intelligibility is rarely the only reason for calling a different langauge different. E.g the scandinavian langauges are relatively easily mutually intelligible, so is Czech and Slovak, yet they are called different languages mainly for political reasons. On the other hand, many of the Chinese "dialects" are not even alike, yet they are still considered one language for political reasons. Even perhaps more familiar "Black English" is usually considered to be nothing more than English with bad grammar, even though it's very clearly a distinct language of its own, that is not readily mutually intelligible with standard american English.
You just have a quirky understanding what a language actually is. Language names are given and defined in language themselves, and it does make a world of a difference if you say you speak Swedish, Danish, or Norwegian, even though they are quite MI among each other, and even somewhat MI with German.
Linguists do refer to different languages in China, which are referred to by the government as dialects. So there is much more to it than politics. In your definition probably the whole Indo-Germanic family from Norwegian to Farsi or Hindi would be one language.
Middle-English is referred to as a distinct language which you can study.