Two interesting questions to get to this time.
First off:
“Just between you and I, Mr. McWhorter, why not, I see you today, I seen you yesterday? Because it hasn’t earned its legitimacy? Yet? Changes in usage should percolate very, very slowly up from the trenches. And resistance should be fierce. That’s just between you and I — Harold B. (politically liberal, linguistically conservative).”
The key issue here is writing versus speech. I am not calling for the rules of written language to simply mirror whatever is happening in the spoken vernacular. Frankly, I doubt the world would stop spinning if that happened, but in real life, there are fashions, sentiments, traditions, and I accept them linguistically to a large extent.
As you all likely know, I am just fine with “I see you today / I seen you yesterday” in casual speech. However, I have no interest in arguing that we should write “I seen you yesterday.” Probably most native English speakers have no problem differentiating see, saw and seen if they have to; it’s just that in casual speech, many often don’t.
The extension of seen to marking the simple past is just … drum roll, please!