The NFL draws well compared to other television shows but does not draw well compared to the NFL of the past. Worse, with an additional 70 million more people today (247 million then, compared to 315 million now), the NFL still can't hit their previous numbers.
While the decline of NFL viewership may continue for years, television as a whole is approaching the threshold where it costs more to run a network than the amount of money that can be made from it. Subsidies from governments that are $200+ trillion in the hole, and legalized gambling aren't going to be enough to keep it afloat.
As evidenced by the takeover of streaming and requiring subscriptions of "Sunday ticket" for pay-per-season-viewing, the NFL will be forced fully onto its own medium, leaving the carcass of television behind. As that happens, advertisers will continue to find other options.
https://www.tvfanatic.com/the-streaming-takeover-industry-experts-warn-that-broadcast-and-cable-tv-may-soon-go-extinct/
However, the NFL ratings are UP, year over year, in the past few years. They are the dominant sports league in the country, in the world. They won't be left in the lurch if the Networks go away. Here is a quote from a NYT article about this from Dec of last year:
Austin Karp, the ratings expert from Sports Business Journal, and Anthony Crupi, the viewership expert from Sportico, passed along some updated viewership numbers: NFL games are averaging 17.48 million viewers this season across TV and digital platforms excluding international games. That’s about an 8 percent year-over-year increase from this time last year. “Monday Night Football” is up 24 percent over last year (a direct result of more games on ABC) and Amazon is up 23 percent. NBC is averaging 21.8 million viewers — the best “Sunday Night Football” viewership number at this point since 2015. (All sports have gotten a boost from better accounting for out-of-home viewership in places like bars and restaurants and viewing parties.) What’s worth noting, as Crupi points out, is an NFL viewership increase is an impressive lift given there is an overall 8 percent decline in U.S. TV usage this fall.
In-person attendance has also been UP the past few years.
https://www.statista.com/statistics/193420/regular-season-attendance-in-the-nfl-since-2006/