Through the first 45 days of 2015, American Express (NYSE:AXP) stock has plummeted by more than 15%. As a result, many investors might wonder whether they should run off with their earnings after the stock more than doubled over the last five years, or whether they should buy additional shares now that the company is trading at a more attractive valuation.

There is sensible rationale for both, but only one is the better solution.

Leaving home without it
Earlier this month it was announced that AmEx in March 2016 would end its decade-and-half run as the only credit card accepted at massive discount retailer Costco.

Amex By Daveonflickr

Image Source: Flickr user DaveOnFlickr.

"Taking a very disciplined approach, we began discussions on a possible renewal with Costco well in advance of the contract expiration," said AmEx CEO Ken Chenault when the move was announced. "However, we were unable to reach terms that would have made economic sense for our company and shareholders."

As The Wall Street Journal reported, Costco had the same rationale, as "its breakup with American Express Co. came down to dollars and cents."

The Costco cards account for 10% of AmEx cards in circulation, and in a conference call with investors, Chenault said the company now expects earnings to be either flat or down in 2015, demonstrate modest improvement in 2016, and then regain full steam by 2017.

In addition to this development, a striking report by Bloomberg, headlined "AmEx Is Losing Its Millionaires," said in part:

American Express, long the envy of the industry for its wealthy clientele, is fighting to retain its grip on affluent cardholders like [hedge fund manager, Whitney] Tilson. Rivals including Barclays and JPMorgan Chase are courting them with enhanced perks, lower fees and more incentives. And as AmEx seeks to diversify by pursuing tech-savvy millennials and underbanked Americans, the risk of eroding its brand -- and its biggest source of revenue -- is rising.

The report noted that 59% of wealthy households -- those with more than $1 million in investable assets -- had an American Express card last year -- the same amount as in 2012. However, Chase saw its market presence among the wealthiest rise from 52% in 2012 to 58% in 2014.

Needless to say, investors have every right to be concerned about American Express' situation. But while there has been plenty of bad news, there hasn't been only bad news.

I left home without it? I'm going back to get it
Lost in the headlines is the reality that American Express had an incredible run in fiscal 2015. Earnings per share rose 14% year over year and return on average equity expanded from 27.8% to 29.1%. On a currency-adjusted basis, average card member spending rose 5% year over year, and its total billed business rose by 9% to surpass $1 trillion.

Chenault noted in the fourth-quarter earnings announcement:

We've made very good progress against the backdrop of an uneven global economy and the negative impact of a strengthening U.S. dollar. We see many opportunities for growth, but at the same time, we face competitive and regulatory challenges. Our aim is to continue to capitalize on the opportunities while dealing with the challenges as we enter 2015.

Yes, Barclays, JPMorgan Chase, and other rivals might seek to eat away at AmEx's lead among the wealthiest of households, but American Express is not content to sit idly and watch its profits evaporate. Through initiatives such as the OptBlue program to allow greater acceptance of its cards at smaller merchants, along with the introduction of the no-fee EveryDay card, AmEx is targeting core middle-class customers as it seeks to expand its product offerings and become more inclusive.

The takeaway
The loss of Costco and other partnerships will hurt American Express in the short term. But if AmEx felt that the dynamics of the new deal wouldn't be in the best interest of its shareholders over the long term, then it is likely best that the company walked away. And when you consider it has an entire year to get those customers on to another American Express card, the impact might be minimal.

When it comes to the supposed dilution of its brand by becoming more inclusive, so long as the company does not relinquish efficiency or let the credit quality of its customers deteriorate -- the foundation of profitable banking -- then expanding its customer base will only be a good thing.

American Express has been an incredible investment for decades, and despite investors' recent uncertainty, all signs point to the company continuing on that path for years to come. Investors could cut and run, but they'd likely miss out on what looks like an opportunity to increase their ownership in a great business at a reduced price.

Patrick Morris has no position in any stocks mentioned. The Motley Fool recommends American Express and Costco Wholesale. The Motley Fool owns shares of Costco Wholesale and JPMorgan Chase. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.