Long-term investing is the key to sustainable returns in the stock market. And while millennial-friendly trading app Robinhood is stereotyped by some as a platform for risk-hungry speculators, some of its most widely held stocks are poised for long-term success. Let's explore the reasons why two of them -- PayPal Holdings (PYPL 0.47%) and Amazon (AMZN -0.29%) -- are good enough to buy and hold forever. 

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PayPal: Developing new growth opportunities

Mobile payments are popular -- especially among young people. Consulting group McKinsey estimates that 91% of all millennials have made a mobile payment in the last year, so it's no surprise that PayPal's stock gets a lot of attention on Robinhood. The fintech giant is poised for continued success as it invests in innovative products like Venmo that can unlock new growth opportunities.

PayPal's long-term strategy relies on the digitization of the global economy and the increasing popularity of mobile wallets. According to CEO Dan Schulman, PayPal will benefit from these megatrends because of its trusted brand and scale. The company made its name as the primary payment processor for eBay. And while that relationship ended in 2018, PayPal has transformed itself into a fintech market leader, with 22% of Americans regularly using its services as of 2018, according to financial services company ING. 

Mobile payments

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PayPal's success will depend on its ability to quickly pivot to new opportunities in the fintech space, such as peer-to-peer (P2P) payments. The company's Venmo subsidiary is making a splash in this market -- with payment volume growing 61% to $44.3 billion in the third quarter. Management expects revenue to approach $900 billion by 2021 as Venmo incorporates new features such as merchant payments and a Venmo credit card in the first quarter of fiscal 2021. 

PayPal's companywide transaction volume jumped 36% to $247 billion in the third quarter, while revenue increased by 25% to $5.46 billion. PayPal is consistently profitable, reporting a net income of $1.02 billion, up from $462 million in the prior-year period. This is important because it shows the company's business model is sustainable. 

Amazon: Healthcare could be a new growth driver

Like PayPal, Amazon's business is hugely popular with millennials -- a demographic that does 60% of its shopping online, according to a survey by research group CouponFollow. Amazon dominates U.S. e-commerce and cloud computing with market shares of 49% and 33%, respectively. And it can maintain its growth by investing in last-mile delivery and expanding into new markets such as fitness wearables. 

The coronavirus pandemic has accelerated trends toward digitization in retail. And Amazon is working hard to compete with rivals like Walmart that have inbuilt advantages in last-mile grocery delivery because of their expansive brick-and-mortar footprints. The company plans to open up to 1,000 small delivery hubs across the U.S. to bring products closer to consumers and make delivery as fast as possible. 

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While Amazon is known for its e-commerce dominance, the company has a track record of successfully pivoting into other markets. The best example of this is cloud computing -- which makes up 12% of the company's net sales and over half of its operating income in the third quarter. The next big pivot could be to fitness wearables. 

Amazon recently released a new wearable service called Amazon Halo that uses AI to help customers monitor health and wellness information. While it's still unclear how much revenue Halo will generate, the overall fitness wearable market is expected to grow at a CAGR of almost 14% to $12.4 billion by 2022. Amazon's entry into this market is a sign that management isn't resting on its laurels and continues to explore new growth drivers that can deliver value to investors. 

Betting on digitization 

Digitization is a megatrend that is changing the way people live and do business. Amazon and PayPal are two popular Robinhood stocks that allow investors to bet on this transformation. Both companies look poised for long-term success as they build on their core strengths in e-commerce and mobile payments while expanding into new market opportunities.