The tech-heavy NASDAQ Composite index has lost a lot of ground since it peaked on Feb. 12. Its declines since then have been propelled by rising Treasury yields, inflation concerns, and worries that tech stocks may have risen into bubble territory.

The bad news for investors is that a major stock market correction or full-blown crash may be on the way in 2021. Certainly there are a few solid reasons to expect such a plunge.  And if that occurs, top tech names such as Apple (NASDAQ:AAPL) and Twilio (NYSE:TWLO) -- both down substantially already in recent weeks -- could fall further.

^IXIC Chart

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Investors, however, should view such a selloff as an opportunity: Apple and Twilio are worth buying and holding for the long run, especially if you can pick them up during a dip.

Person pressing buy button on a keyboard.

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Apple's sales momentum is getting stronger

Apple climbed to the top of the smartphone sales chart in the fourth quarter, based on IDC's estimates, as iPhone 12s turned out to be hot commodities. There are no signs of a slowdown in iPhone 12 demand among consumers. The devices have reportedly kicked off a smartphone upgrade supercycle, according to supply chain checks carried out by analysts from Wedbush.

AppleInsider reports that Wedbush analysts haven't seen Apple cutting build orders for the March quarter in recent weeks as iPhone 12 demand has remained strong. Apple has been known to reduce its iPhone manufacturing in the first and second quarters of the calendar year, but it doesn't seem to be doing so this time around.

The company is expected to build between 56 million and 62 million iPhones this quarter, which would be a big jump over the estimated 36.7 million units it sold in the first quarter of 2020. What's more, Apple's iPhone 12 builds for the June quarter could be in the mid-40 million range, indicating that management does not expect demand to ease significantly even as the launch of the smartphone's next iteration approaches.

Analysts believe that Apple could set a new sales record in 2021 with shipments of 250 million iPhones, and that sales momentum is expected to carry over into 2022 as the transition to 5G continues. Apple is expected to place a build order for 100 million units of this year's iPhone as compared to 80 million initial build orders of the iPhone 12 last year.

Apple's strong smartphone sales could continue beyond 2022 as the 5G market is still in its early phases of growth. Gartner estimates that 5G smartphones will account for 35% of overall smartphone sales in 2021, indicating that there will be more room for them to add market share in the coming years. IDC, for instance, predicts that 5G smartphones will account for 69% of total shipments in 2025.

All of this bodes well for Apple as the iPhone 12 quickly became the top-selling 5G smartphone, with Counterpoint Research estimating that the device grabbed a 16% share of the 5G smartphone market shortly after its debut. The iPhone 12 Pro reportedly accounted for 8% of 5G smartphones sold in October.

This dominance of the 5G market isn't surprising, and that momentum should persist, sending Apple's earnings higher.

AAPL EPS Estimates for Current Fiscal Year Chart

AAPL EPS Estimates for Current Fiscal Year data by YCharts

Given these tailwinds, investors should buy any dips in Apple stock. The company is expected to clock more than 20% revenue growth this year, bettering fiscal 2020's growth of just 5%. This makes Apple a great buy right now as it is trading at 7.1 times sales, lower than the 2020 average multiple of nearly 8.5.

Twilio's secular catalysts make it an enticing buy right now

Cloud communications specialist Twilio has experienced a steep share price downturn after a solid start to the year, despite a solid fiscal 2020 earnings report.

But Twilio's guidance for the current quarter indicates that it is on track to sustain a terrific pace of growth. The company expects top-line growth of 45.5% in the current quarter after finishing fiscal 2020 with a revenue increase of 55%. Also, it is worth noting that it had originally expected 30% to 31% revenue growth for 2020, and it ended up blasting past those expectations due to the tailwinds created by the pandemic.

It won't be surprising to see Twilio exceed its own expectations once again in 2021 as more companies make the switch to cloud-based contact centers. According to Twilio's 2021 state of consumer engagement report, 95% of the 2,500 enterprise decision-makers surveyed expect to either maintain or increase their investment in customer engagement, and 87% believe that digital engagement will play an important role in their business.

What's more, 92% of the businesses Twilio surveyed said that the COVID-19 pandemic had led them to accelerate their shift to the cloud. Twilio anticipates this digital transformation will continue. It points out that IDC has forecast that annual investments in digital transformation will double by 2023 to $2.3 trillion and account for over half of information technology spending. As such, it won't be surprising to see more call center seats move to the cloud.

Twilio has estimated that only 17% of the 15 million contact center seats were in the cloud before the pandemic. That proportion is expected to jump to 50% by 2025, setting the stage for the company to sustain its high rate of growth for years to come.

Additionally, it has been diversifying into higher-margin businesses with the help of acquisitions that should help support long-term margin expansion. The recent purchase of Segment, for instance, will support cross-selling opportunities at Twilio and expand the company's presence into the fast-growing customer data platform market.

Not surprisingly, analysts estimate that Twilio's revenues will grow by more than 30% annually over the next couple of years. That trend could continue over a longer period as contact centers move to the cloud. In all, Twilio's stock price declines in recent weeks offer investors a better opportunity to buy shares of a company that appears primed for long-term growth.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.