You've all pretty well nailed it in analysis, as far as I'm concerned. I thought about all those alternate methods of viewing, and what they're viewing. Another thing that comes into play is that TV ratings are always pretty well focused on what's available through free air TV, cable, and satellite. Alternate viewing sources, such as "on demand," etc., don't seem to be brought into the equation. I say that, but I'm not certain about the last sentence, because it never seems to be spelled out if it's included or not, in their estimations.
One of the things I'd like to see is an actual year by year breakdown of new, renewed, and total subscribers to service Sunday Ticket, with it broken down by state, and even country, since it's available through (I believe) Shaw in Canada, for our Canadian friends. Their cost was $99, but has risen to $199. There are also avenues being used to watch games in Europe, and other areas of the world.
So many things have changed over the last 20 years. It's a new world out there when it comes to viewing habits.