Very few people are undecided. I would even go further by saying majority of people who say they are undecided have actually decided but like the undecided label. Viewership numbers are not surprising at all. People should already know what they are getting from each candidate.
I watched for pure entertainment purposes and to see how unhinged Trump was going to get. I was not surprised he was unhinged, but I am surprised he got there in the first 60 seconds of the debate.
Biden could have said the word "poop" the entire time and it would not have changed my vote. Just like Trumps blatant racism is not going to change the vote of his base.