Laws of language change

I (frequently) remark that there are two laws of language change: 1) language changes, and 2) you can't do anything about rule number 1. I think I need to add a third: 3) language percolates up rather than trickling down.
The "up" and "down" are social/economic/power/education indicators. Those who are on the top end of the spectrum don't really want language to change. They are happy with it the way it is, and have invested considerable time and energy to mastering the nuances of language that mark them as top drawer.
Consider, for instance, the whole dreary who/whom thing. Exactly when do you use whom? The rule is difficult to elucidate and complex in execution. I can do it, mostly, but then, I've spent my entire adult life studying such things.
There are people (gasp) who have not the time, inclination, or energy for such linguistic shenanigans. These people, when faced with a who/whom dilemma, do one of three things: 1) use who in all cases, 2) substitute that for who, or 3) leave who out altogether when they can.
So, the sentence
I know the man whom my sister loves
Becomes
I know the man who my sister loves
I know the man that my sister loves
I know the man my sister loves
The first of these three is still frowned on in some circles, but the second and third have become standard usage.
Voila! Language change.