It's clear that consumers are ditching Diet Coke. The question now is whether investors should ditch the stock.
According to the trade publication Beverage Digest, Diet Coke sales fell 6.6 percent in 2014, ceding second place in the soda rankings to Pepsi (which saw volume fall 1.4 percent). First place remained Coke, which saw volume dip 1.1 percent. Soda sales overall declined for the 10th straight year.
As Coca-Cola faces what appear to be significant secular headwinds (not to mention adverse currency impacts from the soaring dollar) the stock is flat over the past two years, badly underperforming equities as a whole.
"Coca-Cola investors should be worried," said Dave Seaburg, head of equity sales trading at Cowen & Co. "I don't see [soda sales] coming back, and the indicators are saying it's not necessarily going to come back. ... I think the stock should be on the sidelines."
Analyst say the specific problem with Diet Coke is widespread public concern over the artificial sweetener aspartame as well as a general trend toward "naturalness" in food.
Indeed, if Coca-Cola can capitalize off of that trend, it may not be in such bad shape, contends JPMorgan analyst John Faucher.
"Diets are going to continue to decline, but they should be able to offset most of that decline as long as they have truly healthy beverages—that's what these consumers want," Faucher said Friday on CNBC's "Power Lunch. "
Another potential issue could be marketing. Nik Modi, who covers beverage stocks for RBC Capital Markets, believes that the Coke story is set to improve a bit this year as the company boosts its marketing efforts.
In other words, he doesn't think the soda space is dying.
Pointing out that much smaller competitors Mountain Dew and Red Bull logged 7 percent and 6 percent gains in 2014, respectively, Modi noted, "Last time I checked, those are sodas."
The declines shown by big sodas are "kind of an enigma—I don't have a good answer. All I'm saying is, there's evidence to suggest that sugar water with bubbles can do OK."
And even if sodas are facing severe secular headwinds, they may nonetheless be "well-discounted in the stocks," as Sanford Bernstein analyst Ali Dibadj maintains. He says it will ultimately come down to pricing.
"We believe this is an inflection point in the sector where volume continues down at the same pace but pricing is positive," Dibadj told CNBC.
For the company's part, Coca-Cola appears ready to aggressively attempt a Diet Coke turnaround, telling CNBC: "Diet Coke performance has been improving slightly, but we still have more work to do. We're committed to doing that and committed to getting the brand back on the road to sales growth again. Central to that is the positive reinforcement of brand equities and brand attributes like taste, refreshment and uplift."
Whether Coca-Cola shares will see uplift of their own, however, is an open question.