Illiteracy is the inability to read. It has nothing to do with grammatical constructions.
Bad grammar? Certainly there are conventions to which certain classes adhere at certain times. If one wishes to blend into or to join those classes, one must speak their vernacular. That changes over time, as well as geographically. "Bad grammar" is typically established by a higher class as a way to distinguish itself from the classes beneath it. It has nothing to do with getting a point across, which is the chief function of language. It's all about structure, and structure changes all the time. Otherwise, we would still decline our nouns instead of using prepositions.
You advocate for a standard language, in the model of French, which has a language police called the Académie Française. The Académie tells people which words and what linguistic constructions they may use. French is (officially) much more precise and pure than English. English is much more free-wheeling. If you want to fix English structure and spelling in time and space, good luck. You'll fail, or English will die.
Consider this example of bad grammar, and worse spelling, by our standards:
Whan that aprill with his shoures soote
The droghte of march hath perced to the roote,
And bathed every veyne in swich licour
Of which vertu engendred is the flour;
Whan zephirus eek with his sweete breeth
Inspired hath in every holt and heeth
Tendre croppes, and the yonge sonne
Hath in the ram his halve cours yronne,
And smale foweles maken melodye,
That slepen al the nyght with open ye
(so priketh hem nature in hir corages);
Thanne longen folk to goon on pilgrimages,
And palmeres for to seken straunge strondes,
To ferne halwes, kowthe in sondry londes;
And specially from every shires ende
Of engelond to caunterbury they wende,
The hooly blisful martir for to seke,
That hem hath holpen whan that they were seeke.
I still wouldn't throw Chaucer in the dustbin.