Whatever a grammar of a language is, it is largely impervious to human intervention. That is, the really interesting rules and principles are so basic that we cannot do anything at all about them. What we can do is try to influence some of the minor outcomes, for example, try to insist that people say I drank instead of I drunk or It's I instead of It's me. Essentially that is tinkering with matters of no linguistic consequence. To elevate the study of grammar to the task of trying to bring about "correction" in such matters is to trivialize that study. These matters may be of social consequence and often are, but that is a social observation and not a linguistic one, because I drunk and It's me are linguistically on a par with I drank and It's I. Furthermore, it is an observation that tells us much about social organization and the function of trivia in such organization and nothing about the structure of language.
So what are these basic rules and principles that form the really interesting part of grammar? I think they might be things like the following.
Why is 2 ok
1 I gave a present to him.
2 I gave him a present.
but 4 is not?
3 I explained the problem to him.
4 *I explained him the problem.
Why is the position of adverbs in a sentence relatively free, but we can't put the adverb between the verb and the object?
5 I explained the problem to him clearly.
6 I clearly explained the problem to him.
7 I explained the problem clearly to him.
8 *I explained clearly the problem to him.
In these sentences
9 When I get home, he will be cooking dinner.
10 *When I will get home, he will be cooking dinner.
both clauses describe events in the future, but the verb in the when clause cannot take will. Why?
I think this is why Wardhaugh says that grammar is largely impervious to intervention. We don't even think about changing rules like these. We can try to change the "minor outcomes", but they are a very small part of the overall grammar of English.
I think this also helps explain why using "bad grammar" won't lead to the collapse of society or the collapse of English, or to a lack of clear communication. We might disagree on the minor outcomes - some use less with count nouns, some don't. Some say I drunk and some say I drank. Some say Bathsheba gave Jocko and me the ball and some say Bathsheba gave Jocko and I the ball. Some say it's me and some say it's I. There is a lot of variation, but most of the basic rules and principles are the same for every English speaker.