This article originally appeared on Forbes.com.

Big tech marches on. Retail banking is being upended not by the fintech start-ups that once appeared to be the major threat, but by established technology firms such as Amazon and Apple. Amazon Cash, for instance, lets customers deposit cash directly into their Amazon accounts from more than 10,000 retail locations. The company also loaned more than $1 billion in the past year to small merchants selling through its online platform.

Banks appear to be vulnerable to further incursions. Consider that U.S. consumers ranked PayPal and Amazon nearly as high as banks for trust with their money, in Bain & Company's new survey of more than 133,000 banking customers in 22 countries. Some 55% of US consumers are open to buying financial products from established tech firms. And demand for alternatives to traditional banks will only grow, as younger respondents showed the greatest willingness to try these offerings.

The perils of digital lag

Banks have a stiff challenge when it comes to meeting customer expectations for digital tools. Consumers young and old prefer using websites and mobile applications for their routine banking transactions, which tend to be the most frequent interactions. Refining digital channels to be convenient, multifunctional and easy to use with just a few taps or strokes will be critical to earning customers' advocacy and loyalty. Yet on these basic characteristics, banks' mobile apps and websites fall short in the view of many respondents. Only about half of U.S. respondents said their primary bank's website lets them do everything they need, or is easy to use.

Beyond the basics, banks have barely touched some technologies that have reached a tipping point in consumer markets. More than one-quarter of U.S. respondents would consider using voice-controlled assistants for their everyday banking, and Amazon currently dominates the voice-enabled speaker market with its Alexa assistant on the Echo device. Given that Amazon and others already provide payment services, credit cards and loans, it's plausible that they could offer a suite of retail banking services in the near future.

A few banks have made progress in this regard. Last year, Capital One launched functions using Alexa skills, which employ a chatbot that works with Alexa. These chatbots have some constraints, such as requiring explicit commands—for example, "What's my payoff quote?" for an auto loan. But USAA recently introduced an Alexa offering that lets members speak conversationally.

Beyond the loyalty benefits, banks have compelling cost reasons to accelerate development of digital channels that can handle more routine transactions, moving them out of expensive branches and call centers. The opportunity is huge: For instance, 40% of U.S. respondents went to a branch teller at least once in the previous quarter to make a deposit, compared with 21% using digital channels and 18% using ATMs.

The lower costs and high customer advocacy for digital channels serving routine interactions create a virtuous circle. The more transactions that digital channels accommodate seamlessly, the more customers will use them instead of the call center or branch. And the reduction in "bad" call and branch volumes allows banks to reinvest more to further improve digital channels, while reserving their employees for the knottiest problems.

Cost implications also show up in nonroutine events like disputing fees. There's a significant gap between the leading and the lagging banks in the incidence of these dispute events. Laggards not only suffer from having more detractors among customers as a result, they also bear roughly twice the cost of resolving the dispute as the leaders do. In some cases, this can exceed the revenues collected from the fees themselves. It's better to eliminate such events in the first place.

You have to ask for the sale

The other major untapped opportunity for U.S. banks lies in selling financial products. Hidden defection—when customers purchase an additional product from a competing bank—accounts for 46% of all additional product sales. Credit cards, loans and investments are the most purchased categories at competitors, whereas the primary bank tends to keep a higher share of low-value deposit accounts.

To stem the tide of hidden defections, our analysis of the survey data shows that banks should consider more, and more targeted, marketing and sales pitches. Some 42% of U.S. defectors said they bought from a competitor bank because they received an offer or saw an advertisement. Only one-fifth were actively researching when they decided to buy the product. Just as striking, more than half would have purchased from their primary bank if the bank had made an offer.

The lesson: Ask for the sale. Banks have copious data on their customers' risk profiles and stages of life, which allows them to present the right offer at the right time and place—before competitors step in and poach a willing customer. Since consumers tend to purchase credit cards, investments and other secondary products more frequently through digital channels than they do when buying primary products, it follows that banks should ramp up their digital marketing efforts.

Banks' efforts to build out websites and mobile apps have made for an intense contest within the sector. An entry by Amazon or other tech firms into core banking services would take the competition into a different league. Consumers' expectations keep rising as people grow accustomed to simple, convenient digital channels in other parts of their lives. If banks don't reorient their approach and radically accelerate their rate of progress, loyalty will suffer, and they will watch technology firms poach more business.

Gerard du Toit and Maureen Burns are partners in Bain & Company's Financial Services practice.