Investing in an NFL stadium in downtown Minneapolis is starting to look a little like buying RCA stock in the summer of 1929, gold in 1980 or shares of Apple stock last May.

We decided to sink a half-billion tax dollars into a gleaming U.S. Bank Stadium that finally opened the very season the audience for NFL games started slipping. So we were buying professional football at the peak of the market. What chumps.

Last Monday night our beloved Vikings looked bad in Chicago while losing to a team that previously had just one win. But the real story here is that the game generated the lowest TV rating for a Monday night game in the eighth week of the season since ESPN acquired rights to these games before the 2006 season.

In fact the rating was down 18 percent compared with the same week last year, although it is not really fair to blame just the Vikings (or Bears) for this one. That is more or less in line with the audience declines for NFL games that have been going on all season.

An NFL Sunday evening game broadcast two weeks ago did show a gain in viewers from the week before, yet it was hardly a good-news story for the league. For one thing, the rules of the game have been set up to minimize the chances of the dissatisfying outcome of a tie game, yet the Arizona Cardinals and Seattle Seahawks pulled off a fan-punishing, dreadful tie that concluded with each team blowing easy chances to win.

The other sobering aspect of this game for league executives is that even this matchup of elite teams managed to produce a lower TV rating than the same Sunday evening game of the 2015 season.

Fans might shrug off news of shrinking TV audiences as an advertiser's problem, yet there is no mistaking that what is at risk here is the basic business model of their favorite NFL team. The real value doesn't come from selling tickets (that's entertainment, a different business) but by delivering their fans to advertisers.

Forbes last estimated the Minnesota Vikings' annual revenue at a bit more than $300 million. It is known from the annual financial statements of the publicly held Green Bay Packers, among other sources, that the "national revenue" from the league for each team is about $225 million, and by far most of that is from TV. Much of the remaining local revenue is advertising, too.

To paraphrase the legendary Packers coach Vince Lombardi, advertising for the NFL isn't everything. It's the only thing.

As told in Wall Street Journal reporter Matthew Futterman's excellent book on professional sports, "Players," NFL team owners had played the TV game far better than their peers in other sports. The baseball and basketball owners turned to cable, which paid a lot of bills but helped spoil the appeal of their sport for younger fans in particular, who were unwilling to sign up for expensive cable TV service.

The NFL, on the other hand, has relatively few of its games exclusively on cable TV. For big events like the Super Bowl, the broadcast TV audience has swelled to more than 110 million people. That past success has left close observers of the league struggling to explain how the huge football TV audience could have declined by double digits in just one year.

One factor, however, could be flagging interest in the so-called daily fantasy sports games available from sites like FanDuel. Many fans have apparently decided they don't need to watch so many football games only to jump on the fantasy sports site to quickly lose their money to sharps. Another factor might be fan reaction to a San Francisco 49ers player who initiated a protest that had him taking a knee during the national anthem.

The venerable trade publication Advertising Age, in sifting through the reasons offered for the ratings decline, cut through the clutter and just went with the simplest explanation — the games aren't worth watching on TV anymore.

Maybe it is the case where more viewers have concluded that what is supposed to be the controlled violence of football has gotten out of hand. The league's last most valuable player, Cam Newton of the Carolina Panthers, certainly seems to think it has. He publicly wondered last weekend why the league doesn't do more to protect him from illegal and savage hits.

Others point the finger at the game officials, calling penalty after penalty for things like taunting. The Oakland Raiders set an NFL record with 23 penalties in one particularly ugly game last weekend. That means play stopped 23 times to have a game official turn to the camera, flip on his mic, and explain what a player did wrong. Who besides a die-hard Raiders fan would waste a day watching that on TV?

A decline in the TV audience as big as this year's certainly has to have the attention of league officials. It has happened before that broadcasters renewed their rights to show NFL games for less money than they paid on the previous contract. If the pot of TV money shared by the teams actually starts shrinking, it will put pressure on them to try to squeeze more money from the local market, from local advertisers and from fans hopefully willing to let an afternoon's amusement turn into a four-figure family expense.

That will be a bigger challenge if the declining TV audience means the cultural relevance of the entire sport starts to fade, here and everywhere else. Can't imagine how selling anything NFL-related will be easier when yesterday's game has become less likely to be the buzz at work Monday morning and fewer people even notice which musical act just got announced for the halftime show at the Super Bowl.

For fans convinced this talk of moving past peak NFL football is defeatist nonsense, there is a way to put some money behind that conviction. At U.S. Bank Stadium the most expensive of the seat licenses, those intangible assets that grant the right to buy a game ticket, were listed at $56,250 per seat on PSL Source last week. That will buy the privilege to purchase a ticket in the eighth row at about the 50-yard line.

That wouldn't be a financial risk right now that I would be willing to take. 612-673-4302