Some things already don't.
Per capita consumption of full-calorie soft drinks fell by more than 20 percent from 2000 to 2011, according to Beverage Digest. Diet Coke is now No. 2, ahead of Pepsi.
So how far can regular soda fall, and how fast?
Will it go back to being the treat that it was 50 years ago, when vending machines sold 6.5-ounce bottles? Will soda companies find their Holy Grail - a natural, no-calorie sweetener that tastes just like sugar, erasing the line between diet and regular?
"In the very long term, sugar-sweetened beverages, and probably juices, will be regulated in the U.S. just like cigarettes," said Barry M. Popkin, an economist and nutrition professor at the University of North Carolina-Chapel Hill. He said rising health costs that hurt American competitiveness will be a factor.
Comparisons between soda and tobacco are imperfect. Even second-hand smoke is harmful, whereas soda is more a matter of too many calories coming in for the amount going out. But there are intriguing parallels.
Advertising played a big role in the rise of each - from the American Tobacco Co.'s "Reach for a Lucky Instead of a Sweet" campaign in the early 1930s to the airwaves-saturating battle between Coke and Pepsi a half-century later.
In both cases, education - the surgeon general's report on smoking in 1964 and various health initiatives on obesity in recent years - coincided with the end of long-term trends.
"But it wasn't until you saw the policy changes" - cigarette taxes, smoking bans, and advertising restrictions - "where you actually saw decreases" in smoking rates, said Mary Story, who studies obesity prevention at the University of Minnesota.
There has been no surgeon general's report on soda, although nearly 100 health groups called for one in July.
So far, the scientific evidence linking soda and obesity falls into three categories:
For years, people drank more and more soda. Daily caloric intake of sugar-sweetened beverages nearly tripled between 1977 and 2001, Popkin found, roughly paralleling the surge in obesity. Teenage boys now get more calories from sugar-sweetened beverages than from any other food or drink.
Liquid calories don't make you feel full in the way that solid food does, so the body doesn't compensate for them by eating less, a growing body of research has found.
Emerging evidence suggests that high-fructose corn syrup, a mainstay of soda, might be metabolized differently from other substances.
If soda is on a tobacco trajectory, said Kelly D. Brownell, director of Yale's Rudd Center for Food Policy and Obesity, then "everything is happening at a very accelerated rate."
In less than a decade of scrutiny, sugar-sweetened beverages have been removed from most schools nationwide; portions are set to be cut by law in New York; calories are labeled by mandate in such cities as Philadelphia and will be voluntarily in Chicago and San Antonio, Pepsico and Coca-Cola said this month; and various soda taxes have been proposed, with two more on local ballots in California next month.
Even when the anti-soda lobby loses, as it has with every tax so far, more "information comes out. . . . Usually consumers only hear what the food and beverage companies say," said Harold Goldstein, executive director of the California Center for Public Health Advocacy.
The industry, which the Federal Trade Commission estimated spends nearly $500 million a year marketing sugary beverages to adolescents alone, is light years ahead of public health campaigns. But the public health advocates are improving.
The Real Bears, a cartoon video by the Center for Science in the Public Interest that went viral, shows a family of polar bears surrounded by soft-drink advertising as extra-large Baby Bear gets stuck in an ice fishing hole and Papa Bear experiences erectile dysfunction, a side effect of diabetes. They finally pour their sodas into the sea.
"We're trying to figure out, what are the emotional factors. Does fear work? Does nurturance work? Does humor work?" said Jordan, the Annenberg researcher.
Her findings helped shape a 30-second TV commercial, which is posted at www.foodfitphilly.org, the city health department's sugary drinks site.
The commercial, part of a $1.5 million ad campaign funded by federal stimulus money, shows a mother driving with her son, thinking about what his doctor said about diabetes. "We'll fix this," she says finally, shaking her head at their two big drinks and gazing at her son with concern. "Just wish I'd known sooner."
No one expects soft-drink manufacturers to go under.
"I kind of view the obesity crisis as the indicator of the next big business opportunity for all these companies," said Hank Cardello, a former beverage company CEO and author of Stuffed: An Insider's Look at Who's (Really) Making America Fat. Cardello said smaller portion sizes and "good for you" beverages tend to be more profitable.
The industry is responding to consumer demand, American Beverage Association spokeswoman Karen Hanretty said, by creating smaller containers and new products like vitamin water.
Some have caught on quickly. But diet sodas, while rising four percentage points in market share as full-calorie fell by four points, are not being consumed any more than in 2000. Hence the industry's focus on new formulations.
Within two years, "you will see Pepsi and Coke and Dr Pepper coming up with a whole variety of no-calorie sweeteners," said Harold Honickman, who chairs the region's largest independent bottling group and worked on the beverage association's campaigns that twice gutted Mayor Nutter's proposed soda taxes.
Twenty years from now, Honickman said, "I honestly think you will find 'regular' Pepsi, 'regular' Coke with new kinds of sweeteners. They will be better-tasting drinks than we have today."
And truly regular sodas?
"I personally think that soda will remain a large product category in the U.S. Responsibly consumed, it's not just healthy and fine, it is also fun and tasty," said John Sicher, publisher of the independent Beverage Digest. What that means differs from person to person, he said.
The American Heart Association recommends that most women consume no more than six teaspoons and men nine teaspoons a day of "added sugar" - the stuff added by manufacturers to cookies, ice cream, fruit-filled yogurt, soda, and anything you stir into your own coffee.
A 12-ounce can of Coke contains eight teaspoons of added sugar.
David B. Allison, director of the Nutrition Obesity Research Center at the University of Alabama at Birmingham, has often played a contrarian role on the soda issue, and the industry refers reporters to him for an opposing scientific viewpoint. But he said that two studies published last month in the New England Journal of Medicine led him to conclude there is now sufficient evidence to show reducing consumption of sugar-sweetened beverages will reduce obesity in certain people.
Eventually, he said, drinking a soda will be akin to smoking a cigar or not wearing a seat belt.
"What I say to my kids" - ages 7, 9 and 12 - "is whenever possible, do not have sugar-sweetened beverages," Allison said. "If you are at a birthday party and the only beverage available is a sugar-sweetened beverage, and you want to be comfortable [in the group], if that happens once a month, it's not a problem."
Contact Don Sapatkin at 215-854-2617 or email@example.com.