Week 4 NFL Ratings up in five out of six viewing windows
Not only that, viewership didn't even drop in 2016 and 2017 if you adjust for the fact that ALL TV viewing dropping during those years. The problem isn't the NFL's so much as it is the whole TV industry's - younger people don't watch as much TV as previous younger generations did. Which makes sense, given that there is plenty of stuff on the web, youtube, Fortnite and so forth to occupy one's time that didn't exist a decade ago.
The problem with "OK just offer the games via streaming" is that people aren't going to want to watch a 3 1/2 hour game on their phone. You get like 15 minutes of action and 3 1/4 hours of ads and talking heads. Or maybe they will - one of the fastest growing things on Youtube is watching people play video games. Yes, you read that right, people will watch someone else livestream themselves playing a video game and commenting. If you're willing to sit through that, you will watch almost anything!