The National Football League is a glass house that was cracking well before Donald Trump’s criticism of players who refuse to stand during the national anthem.

The NFL earned an estimated $14 billion last year. But 500-channel television, internet live streaming, video games and all sorts of other televised sports have combined to threaten the league’s monopoly on weekend entertainment — even before recent controversies.

It has become a fad for many players not to stand for the anthem. But it is also becoming a trend for irate fans not to watch the NFL at all.

Multimillionaire young players, mostly in their 20s, often cannot quite explain why they have become so furious at emblems of the country in which they are doing so well.

Their gripes at best seem episodic and are often without supporting data. Are they mad at supposedly inordinate police brutality toward black citizens, or racial disparity caused by bias, or the perceived vulgarity of President Donald Trump?

The result, fairly or not, is that a lot of viewers do not understand why so many young, rich players show such disrespect for their country — and, by extension, insult their far poorer fans, whose loyal support has helped pay their salaries.

ESPN talking heads and network TV analysts do not help. They often pose as social justice warriors, but they are ill-equipped to offer sermons to fans on their ethical shortcomings that have nothing to do with football.

In truth, the NFL’s hard-core fan base is not comprised of bicoastal hipsters. Rather, the league’s fan base is formed mostly by red-state Americans — and many of them are becoming increasingly turned off by the culture of professional football.

Professional athletes are frequently viewed as role models. Yet since 2000, more than 850 NFL players have been arrested, some of them convicted of heinous crimes and abuse against women.

The old idea of quiet sportsmanship — downplaying one’s own achievements while crediting the accomplishments of others — is being overshadowed by individual showboating.

Players are now bigger, faster and harder-hitting than in the past. Research has revealed a possible epidemic of traumatic brain injuries and other crippling injuries among NFL players. Such harm threatens to reduce the pool of future NFL players.

There is a growing public perception that the NFL is less a reflection of the kind of athleticism seen in basketball or baseball, and more a reflection of the violence of Mixed Martial Arts — or of gladiators in the ancient Roman Colosseum.

NFL Commissioner Roger Goodell, who has received more than $212 million in compensation since 2006, has been a big moneymaker for the owners. Yet otherwise, he has been a public relations disaster, due largely to his incompetent efforts to sound politically correct. Goodell often insists that trivial rules be observed to the letter. For instance, the league denied a request by Dallas Cowboys players to wear small decals honoring Dallas police officers killed in a 2016 shooting.

At other times, Goodell deliberately ignores widespread violations of important NFL regulations — like the requirement that all players show respect for the American flag by solemnly standing during the national anthem.

The average value of an NFL franchise is estimated at $2.5 billion. The average player salary is nearly $2 million a year. Some of the league’s superstars are making more than $20 million a year. Given such wealth, local governments are understandingly becoming miffed that they have to pony up public money to support new NFL stadiums. Over the last two decades, the American public has shelled out more than $7 billion to build or renovate NFL stadiums. Billionaire owners are able to blackmail the public to pay to keep a franchise or else lose it to a city that offers bigger stadium subsidies.

Meanwhile, the NFL has successfully lobbied for exemption from federal antitrust regulations. NFL owners are crony capitalists who want the state both to subsidize them and leave them alone.

Racial politics in the NFL have become increasingly problematic. The mega-wealthy franchise owners are almost all white businesspeople. Their multimillionaire players are about 70 percent African-American. So there is little diversity among the players, but even less among the owners.

Politically correct orthodoxy dictates that even quasi-public entities like the NFL should “look like America.” But instead, the NFL apparently sees itself as an old-fashioned meritocracy where athletic skills and corporate success alone determine who plays and who owns.

The progressive notions of “proportional representation” and “disparate impact” that sometimes govern universities and government mysteriously do not apply to the NFL, where few Latinos or Asians are included.

But most importantly, the league has entirely forgotten the fundamental rule of business: Never ignore, insult or talk down to the loyal consumers who provide the leagues’ support and income.

The NFL will not disappear, but its national significance is rapidly diminishing.

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