The main reason for declining viewership is their viewers are dying off with the WWI vets.
Hmmmmmmmmmmm. Let's see...total viewership of 7.16 million, of which 2.18 million was 25-54. Since "Nightly's" viewership among the under 25 demographic is approximately zero, that means that roughly 70% of their audience is over 55.
That's a viewership profile that's doomed to fail over the long run.